import.go

Documentation: cmd/go/internal/modload

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package modload
     6  
     7  import (
     8  	"context"
     9  	"errors"
    10  	"fmt"
    11  	"go/build"
    12  	"internal/goroot"
    13  	"io/fs"
    14  	"os"
    15  	pathpkg "path"
    16  	"path/filepath"
    17  	"sort"
    18  	"strings"
    19  
    20  	"cmd/go/internal/cfg"
    21  	"cmd/go/internal/fsys"
    22  	"cmd/go/internal/modfetch"
    23  	"cmd/go/internal/par"
    24  	"cmd/go/internal/search"
    25  
    26  	"golang.org/x/mod/module"
    27  	"golang.org/x/mod/semver"
    28  )
    29  
    30  type ImportMissingError struct {
    31  	Path     string
    32  	Module   module.Version
    33  	QueryErr error
    34  
    35  	// isStd indicates whether we would expect to find the package in the standard
    36  	// library. This is normally true for all dotless import paths, but replace
    37  	// directives can cause us to treat the replaced paths as also being in
    38  	// modules.
    39  	isStd bool
    40  
    41  	// replaced the highest replaced version of the module where the replacement
    42  	// contains the package. replaced is only set if the replacement is unused.
    43  	replaced module.Version
    44  
    45  	// newMissingVersion is set to a newer version of Module if one is present
    46  	// in the build list. When set, we can't automatically upgrade.
    47  	newMissingVersion string
    48  }
    49  
    50  func (e *ImportMissingError) Error() string {
    51  	if e.Module.Path == "" {
    52  		if e.isStd {
    53  			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
    54  		}
    55  		if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
    56  			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
    57  		}
    58  		if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
    59  			return "cannot find module providing package " + e.Path
    60  		}
    61  
    62  		if e.replaced.Path != "" {
    63  			suggestArg := e.replaced.Path
    64  			if !module.IsZeroPseudoVersion(e.replaced.Version) {
    65  				suggestArg = e.replaced.String()
    66  			}
    67  			return fmt.Sprintf("module %s provides package %s and is replaced but not required; to add it:\n\tgo get %s", e.replaced.Path, e.Path, suggestArg)
    68  		}
    69  
    70  		message := fmt.Sprintf("no required module provides package %s", e.Path)
    71  		if e.QueryErr != nil {
    72  			return fmt.Sprintf("%s: %v", message, e.QueryErr)
    73  		}
    74  		return fmt.Sprintf("%s; to add it:\n\tgo get %s", message, e.Path)
    75  	}
    76  
    77  	if e.newMissingVersion != "" {
    78  		return fmt.Sprintf("package %s provided by %s at latest version %s but not at required version %s", e.Path, e.Module.Path, e.Module.Version, e.newMissingVersion)
    79  	}
    80  
    81  	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
    82  }
    83  
    84  func (e *ImportMissingError) Unwrap() error {
    85  	return e.QueryErr
    86  }
    87  
    88  func (e *ImportMissingError) ImportPath() string {
    89  	return e.Path
    90  }
    91  
    92  // An AmbiguousImportError indicates an import of a package found in multiple
    93  // modules in the build list, or found in both the main module and its vendor
    94  // directory.
    95  type AmbiguousImportError struct {
    96  	importPath string
    97  	Dirs       []string
    98  	Modules    []module.Version // Either empty or 1:1 with Dirs.
    99  }
   100  
   101  func (e *AmbiguousImportError) ImportPath() string {
   102  	return e.importPath
   103  }
   104  
   105  func (e *AmbiguousImportError) Error() string {
   106  	locType := "modules"
   107  	if len(e.Modules) == 0 {
   108  		locType = "directories"
   109  	}
   110  
   111  	var buf strings.Builder
   112  	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
   113  
   114  	for i, dir := range e.Dirs {
   115  		buf.WriteString("\n\t")
   116  		if i < len(e.Modules) {
   117  			m := e.Modules[i]
   118  			buf.WriteString(m.Path)
   119  			if m.Version != "" {
   120  				fmt.Fprintf(&buf, " %s", m.Version)
   121  			}
   122  			fmt.Fprintf(&buf, " (%s)", dir)
   123  		} else {
   124  			buf.WriteString(dir)
   125  		}
   126  	}
   127  
   128  	return buf.String()
   129  }
   130  
   131  // A DirectImportFromImplicitDependencyError indicates a package directly
   132  // imported by a package or test in the main module that is satisfied by a
   133  // dependency that is not explicit in the main module's go.mod file.
   134  type DirectImportFromImplicitDependencyError struct {
   135  	ImporterPath string
   136  	ImportedPath string
   137  	Module       module.Version
   138  }
   139  
   140  func (e *DirectImportFromImplicitDependencyError) Error() string {
   141  	return fmt.Sprintf("package %s imports %s from implicitly required module; to add missing requirements, run:\n\tgo get %s@%s", e.ImporterPath, e.ImportedPath, e.Module.Path, e.Module.Version)
   142  }
   143  
   144  func (e *DirectImportFromImplicitDependencyError) ImportPath() string {
   145  	return e.ImporterPath
   146  }
   147  
   148  // ImportMissingSumError is reported in readonly mode when we need to check
   149  // if a module contains a package, but we don't have a sum for its .zip file.
   150  // We might need sums for multiple modules to verify the package is unique.
   151  //
   152  // TODO(#43653): consolidate multiple errors of this type into a single error
   153  // that suggests a 'go get' command for root packages that transtively import
   154  // packages from modules with missing sums. load.CheckPackageErrors would be
   155  // a good place to consolidate errors, but we'll need to attach the import
   156  // stack here.
   157  type ImportMissingSumError struct {
   158  	importPath                string
   159  	found                     bool
   160  	mods                      []module.Version
   161  	importer, importerVersion string // optional, but used for additional context
   162  	importerIsTest            bool
   163  }
   164  
   165  func (e *ImportMissingSumError) Error() string {
   166  	var importParen string
   167  	if e.importer != "" {
   168  		importParen = fmt.Sprintf(" (imported by %s)", e.importer)
   169  	}
   170  	var message string
   171  	if e.found {
   172  		message = fmt.Sprintf("missing go.sum entry needed to verify package %s%s is provided by exactly one module", e.importPath, importParen)
   173  	} else {
   174  		message = fmt.Sprintf("missing go.sum entry for module providing package %s%s", e.importPath, importParen)
   175  	}
   176  	var hint string
   177  	if e.importer == "" {
   178  		// Importing package is unknown, or the missing package was named on the
   179  		// command line. Recommend 'go mod download' for the modules that could
   180  		// provide the package, since that shouldn't change go.mod.
   181  		if len(e.mods) > 0 {
   182  			args := make([]string, len(e.mods))
   183  			for i, mod := range e.mods {
   184  				args[i] = mod.Path
   185  			}
   186  			hint = fmt.Sprintf("; to add:\n\tgo mod download %s", strings.Join(args, " "))
   187  		}
   188  	} else {
   189  		// Importing package is known (common case). Recommend 'go get' on the
   190  		// current version of the importing package.
   191  		tFlag := ""
   192  		if e.importerIsTest {
   193  			tFlag = " -t"
   194  		}
   195  		version := ""
   196  		if e.importerVersion != "" {
   197  			version = "@" + e.importerVersion
   198  		}
   199  		hint = fmt.Sprintf("; to add:\n\tgo get%s %s%s", tFlag, e.importer, version)
   200  	}
   201  	return message + hint
   202  }
   203  
   204  func (e *ImportMissingSumError) ImportPath() string {
   205  	return e.importPath
   206  }
   207  
   208  type invalidImportError struct {
   209  	importPath string
   210  	err        error
   211  }
   212  
   213  func (e *invalidImportError) ImportPath() string {
   214  	return e.importPath
   215  }
   216  
   217  func (e *invalidImportError) Error() string {
   218  	return e.err.Error()
   219  }
   220  
   221  func (e *invalidImportError) Unwrap() error {
   222  	return e.err
   223  }
   224  
   225  // importFromModules finds the module and directory in the dependency graph of
   226  // rs containing the package with the given import path. If mg is nil,
   227  // importFromModules attempts to locate the module using only the main module
   228  // and the roots of rs before it loads the full graph.
   229  //
   230  // The answer must be unique: importFromModules returns an error if multiple
   231  // modules are observed to provide the same package.
   232  //
   233  // importFromModules can return a module with an empty m.Path, for packages in
   234  // the standard library.
   235  //
   236  // importFromModules can return an empty directory string, for fake packages
   237  // like "C" and "unsafe".
   238  //
   239  // If the package is not present in any module selected from the requirement
   240  // graph, importFromModules returns an *ImportMissingError.
   241  func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph) (m module.Version, dir string, err error) {
   242  	if strings.Contains(path, "@") {
   243  		return module.Version{}, "", fmt.Errorf("import path should not have @version")
   244  	}
   245  	if build.IsLocalImport(path) {
   246  		return module.Version{}, "", fmt.Errorf("relative import not supported")
   247  	}
   248  	if path == "C" {
   249  		// There's no directory for import "C".
   250  		return module.Version{}, "", nil
   251  	}
   252  	// Before any further lookup, check that the path is valid.
   253  	if err := module.CheckImportPath(path); err != nil {
   254  		return module.Version{}, "", &invalidImportError{importPath: path, err: err}
   255  	}
   256  
   257  	// Is the package in the standard library?
   258  	pathIsStd := search.IsStandardImportPath(path)
   259  	if pathIsStd && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
   260  		if targetInGorootSrc {
   261  			if dir, ok, err := dirInModule(path, targetPrefix, ModRoot(), true); err != nil {
   262  				return module.Version{}, dir, err
   263  			} else if ok {
   264  				return Target, dir, nil
   265  			}
   266  		}
   267  		dir := filepath.Join(cfg.GOROOT, "src", path)
   268  		return module.Version{}, dir, nil
   269  	}
   270  
   271  	// -mod=vendor is special.
   272  	// Everything must be in the main module or the main module's vendor directory.
   273  	if cfg.BuildMod == "vendor" {
   274  		mainDir, mainOK, mainErr := dirInModule(path, targetPrefix, ModRoot(), true)
   275  		vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
   276  		if mainOK && vendorOK {
   277  			return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: []string{mainDir, vendorDir}}
   278  		}
   279  		// Prefer to return main directory if there is one,
   280  		// Note that we're not checking that the package exists.
   281  		// We'll leave that for load.
   282  		if !vendorOK && mainDir != "" {
   283  			return Target, mainDir, nil
   284  		}
   285  		if mainErr != nil {
   286  			return module.Version{}, "", mainErr
   287  		}
   288  		readVendorList()
   289  		return vendorPkgModule[path], vendorDir, nil
   290  	}
   291  
   292  	// Check each module on the build list.
   293  	var dirs []string
   294  	var mods []module.Version
   295  
   296  	// Iterate over possible modules for the path, not all selected modules.
   297  	// Iterating over selected modules would make the overall loading time
   298  	// O(M × P) for M modules providing P imported packages, whereas iterating
   299  	// over path prefixes is only O(P × k) with maximum path depth k. For
   300  	// large projects both M and P may be very large (note that M ≤ P), but k
   301  	// will tend to remain smallish (if for no other reason than filesystem
   302  	// path limitations).
   303  	//
   304  	// We perform this iteration either one or two times. If mg is initially nil,
   305  	// then we first attempt to load the package using only the main module and
   306  	// its root requirements. If that does not identify the package, or if mg is
   307  	// already non-nil, then we attempt to load the package using the full
   308  	// requirements in mg.
   309  	for {
   310  		var sumErrMods []module.Version
   311  		for prefix := path; prefix != "."; prefix = pathpkg.Dir(prefix) {
   312  			var (
   313  				v  string
   314  				ok bool
   315  			)
   316  			if mg == nil {
   317  				v, ok = rs.rootSelected(prefix)
   318  			} else {
   319  				v, ok = mg.Selected(prefix), true
   320  			}
   321  			if !ok || v == "none" {
   322  				continue
   323  			}
   324  			m := module.Version{Path: prefix, Version: v}
   325  
   326  			needSum := true
   327  			root, isLocal, err := fetch(ctx, m, needSum)
   328  			if err != nil {
   329  				if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
   330  					// We are missing a sum needed to fetch a module in the build list.
   331  					// We can't verify that the package is unique, and we may not find
   332  					// the package at all. Keep checking other modules to decide which
   333  					// error to report. Multiple sums may be missing if we need to look in
   334  					// multiple nested modules to resolve the import; we'll report them all.
   335  					sumErrMods = append(sumErrMods, m)
   336  					continue
   337  				}
   338  				// Report fetch error.
   339  				// Note that we don't know for sure this module is necessary,
   340  				// but it certainly _could_ provide the package, and even if we
   341  				// continue the loop and find the package in some other module,
   342  				// we need to look at this module to make sure the import is
   343  				// not ambiguous.
   344  				return module.Version{}, "", err
   345  			}
   346  			if dir, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
   347  				return module.Version{}, "", err
   348  			} else if ok {
   349  				mods = append(mods, m)
   350  				dirs = append(dirs, dir)
   351  			}
   352  		}
   353  
   354  		if len(mods) > 1 {
   355  			// We produce the list of directories from longest to shortest candidate
   356  			// module path, but the AmbiguousImportError should report them from
   357  			// shortest to longest. Reverse them now.
   358  			for i := 0; i < len(mods)/2; i++ {
   359  				j := len(mods) - 1 - i
   360  				mods[i], mods[j] = mods[j], mods[i]
   361  				dirs[i], dirs[j] = dirs[j], dirs[i]
   362  			}
   363  			return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
   364  		}
   365  
   366  		if len(sumErrMods) > 0 {
   367  			for i := 0; i < len(sumErrMods)/2; i++ {
   368  				j := len(sumErrMods) - 1 - i
   369  				sumErrMods[i], sumErrMods[j] = sumErrMods[j], sumErrMods[i]
   370  			}
   371  			return module.Version{}, "", &ImportMissingSumError{
   372  				importPath: path,
   373  				mods:       sumErrMods,
   374  				found:      len(mods) > 0,
   375  			}
   376  		}
   377  
   378  		if len(mods) == 1 {
   379  			return mods[0], dirs[0], nil
   380  		}
   381  
   382  		if mg != nil {
   383  			// We checked the full module graph and still didn't find the
   384  			// requested package.
   385  			var queryErr error
   386  			if !HasModRoot() {
   387  				queryErr = ErrNoModRoot
   388  			}
   389  			return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr, isStd: pathIsStd}
   390  		}
   391  
   392  		// So far we've checked the root dependencies.
   393  		// Load the full module graph and try again.
   394  		mg, err = rs.Graph(ctx)
   395  		if err != nil {
   396  			// We might be missing one or more transitive (implicit) dependencies from
   397  			// the module graph, so we can't return an ImportMissingError here — one
   398  			// of the missing modules might actually contain the package in question,
   399  			// in which case we shouldn't go looking for it in some new dependency.
   400  			return module.Version{}, "", err
   401  		}
   402  	}
   403  }
   404  
   405  // queryImport attempts to locate a module that can be added to the current
   406  // build list to provide the package with the given import path.
   407  //
   408  // Unlike QueryPattern, queryImport prefers to add a replaced version of a
   409  // module *before* checking the proxies for a version to add.
   410  func queryImport(ctx context.Context, path string, rs *Requirements) (module.Version, error) {
   411  	// To avoid spurious remote fetches, try the latest replacement for each
   412  	// module (golang.org/issue/26241).
   413  	if index != nil {
   414  		var mods []module.Version
   415  		for mp, mv := range index.highestReplaced {
   416  			if !maybeInModule(path, mp) {
   417  				continue
   418  			}
   419  			if mv == "" {
   420  				// The only replacement is a wildcard that doesn't specify a version, so
   421  				// synthesize a pseudo-version with an appropriate major version and a
   422  				// timestamp below any real timestamp. That way, if the main module is
   423  				// used from within some other module, the user will be able to upgrade
   424  				// the requirement to any real version they choose.
   425  				if _, pathMajor, ok := module.SplitPathVersion(mp); ok && len(pathMajor) > 0 {
   426  					mv = module.ZeroPseudoVersion(pathMajor[1:])
   427  				} else {
   428  					mv = module.ZeroPseudoVersion("v0")
   429  				}
   430  			}
   431  			mg, err := rs.Graph(ctx)
   432  			if err != nil {
   433  				return module.Version{}, err
   434  			}
   435  			if cmpVersion(mg.Selected(mp), mv) >= 0 {
   436  				// We can't resolve the import by adding mp@mv to the module graph,
   437  				// because the selected version of mp is already at least mv.
   438  				continue
   439  			}
   440  			mods = append(mods, module.Version{Path: mp, Version: mv})
   441  		}
   442  
   443  		// Every module path in mods is a prefix of the import path.
   444  		// As in QueryPattern, prefer the longest prefix that satisfies the import.
   445  		sort.Slice(mods, func(i, j int) bool {
   446  			return len(mods[i].Path) > len(mods[j].Path)
   447  		})
   448  		for _, m := range mods {
   449  			needSum := true
   450  			root, isLocal, err := fetch(ctx, m, needSum)
   451  			if err != nil {
   452  				if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
   453  					return module.Version{}, &ImportMissingSumError{importPath: path}
   454  				}
   455  				return module.Version{}, err
   456  			}
   457  			if _, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
   458  				return m, err
   459  			} else if ok {
   460  				if cfg.BuildMod == "readonly" {
   461  					return module.Version{}, &ImportMissingError{Path: path, replaced: m}
   462  				}
   463  				return m, nil
   464  			}
   465  		}
   466  		if len(mods) > 0 && module.CheckPath(path) != nil {
   467  			// The package path is not valid to fetch remotely,
   468  			// so it can only exist in a replaced module,
   469  			// and we know from the above loop that it is not.
   470  			return module.Version{}, &PackageNotInModuleError{
   471  				Mod:         mods[0],
   472  				Query:       "latest",
   473  				Pattern:     path,
   474  				Replacement: Replacement(mods[0]),
   475  			}
   476  		}
   477  	}
   478  
   479  	if search.IsStandardImportPath(path) {
   480  		// This package isn't in the standard library, isn't in any module already
   481  		// in the build list, and isn't in any other module that the user has
   482  		// shimmed in via a "replace" directive.
   483  		// Moreover, the import path is reserved for the standard library, so
   484  		// QueryPattern cannot possibly find a module containing this package.
   485  		//
   486  		// Instead of trying QueryPattern, report an ImportMissingError immediately.
   487  		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
   488  	}
   489  
   490  	if cfg.BuildMod == "readonly" && !allowMissingModuleImports {
   491  		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
   492  		// the module. If readonly mode was enabled explicitly, include that in
   493  		// the error message.
   494  		var queryErr error
   495  		if cfg.BuildModExplicit {
   496  			queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
   497  		} else if cfg.BuildModReason != "" {
   498  			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
   499  		}
   500  		return module.Version{}, &ImportMissingError{Path: path, QueryErr: queryErr}
   501  	}
   502  
   503  	// Look up module containing the package, for addition to the build list.
   504  	// Goal is to determine the module, download it to dir,
   505  	// and return m, dir, ImpportMissingError.
   506  	fmt.Fprintf(os.Stderr, "go: finding module for package %s\n", path)
   507  
   508  	mg, err := rs.Graph(ctx)
   509  	if err != nil {
   510  		return module.Version{}, err
   511  	}
   512  
   513  	candidates, err := QueryPackages(ctx, path, "latest", mg.Selected, CheckAllowed)
   514  	if err != nil {
   515  		if errors.Is(err, fs.ErrNotExist) {
   516  			// Return "cannot find module providing package […]" instead of whatever
   517  			// low-level error QueryPattern produced.
   518  			return module.Version{}, &ImportMissingError{Path: path, QueryErr: err}
   519  		} else {
   520  			return module.Version{}, err
   521  		}
   522  	}
   523  
   524  	candidate0MissingVersion := ""
   525  	for i, c := range candidates {
   526  		if v := mg.Selected(c.Mod.Path); semver.Compare(v, c.Mod.Version) > 0 {
   527  			// QueryPattern proposed that we add module c.Mod to provide the package,
   528  			// but we already depend on a newer version of that module (and that
   529  			// version doesn't have the package).
   530  			//
   531  			// This typically happens when a package is present at the "@latest"
   532  			// version (e.g., v1.0.0) of a module, but we have a newer version
   533  			// of the same module in the build list (e.g., v1.0.1-beta), and
   534  			// the package is not present there.
   535  			if i == 0 {
   536  				candidate0MissingVersion = v
   537  			}
   538  			continue
   539  		}
   540  		return c.Mod, nil
   541  	}
   542  	return module.Version{}, &ImportMissingError{
   543  		Path:              path,
   544  		Module:            candidates[0].Mod,
   545  		newMissingVersion: candidate0MissingVersion,
   546  	}
   547  }
   548  
   549  // maybeInModule reports whether, syntactically,
   550  // a package with the given import path could be supplied
   551  // by a module with the given module path (mpath).
   552  func maybeInModule(path, mpath string) bool {
   553  	return mpath == path ||
   554  		len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
   555  }
   556  
   557  var (
   558  	haveGoModCache   par.Cache // dir → bool
   559  	haveGoFilesCache par.Cache // dir → goFilesEntry
   560  )
   561  
   562  type goFilesEntry struct {
   563  	haveGoFiles bool
   564  	err         error
   565  }
   566  
   567  // dirInModule locates the directory that would hold the package named by the given path,
   568  // if it were in the module with module path mpath and root mdir.
   569  // If path is syntactically not within mpath,
   570  // or if mdir is a local file tree (isLocal == true) and the directory
   571  // that would hold path is in a sub-module (covered by a go.mod below mdir),
   572  // dirInModule returns "", false, nil.
   573  //
   574  // Otherwise, dirInModule returns the name of the directory where
   575  // Go source files would be expected, along with a boolean indicating
   576  // whether there are in fact Go source files in that directory.
   577  // A non-nil error indicates that the existence of the directory and/or
   578  // source files could not be determined, for example due to a permission error.
   579  func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFiles bool, err error) {
   580  	// Determine where to expect the package.
   581  	if path == mpath {
   582  		dir = mdir
   583  	} else if mpath == "" { // vendor directory
   584  		dir = filepath.Join(mdir, path)
   585  	} else if len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath {
   586  		dir = filepath.Join(mdir, path[len(mpath)+1:])
   587  	} else {
   588  		return "", false, nil
   589  	}
   590  
   591  	// Check that there aren't other modules in the way.
   592  	// This check is unnecessary inside the module cache
   593  	// and important to skip in the vendor directory,
   594  	// where all the module trees have been overlaid.
   595  	// So we only check local module trees
   596  	// (the main module, and any directory trees pointed at by replace directives).
   597  	if isLocal {
   598  		for d := dir; d != mdir && len(d) > len(mdir); {
   599  			haveGoMod := haveGoModCache.Do(d, func() interface{} {
   600  				fi, err := fsys.Stat(filepath.Join(d, "go.mod"))
   601  				return err == nil && !fi.IsDir()
   602  			}).(bool)
   603  
   604  			if haveGoMod {
   605  				return "", false, nil
   606  			}
   607  			parent := filepath.Dir(d)
   608  			if parent == d {
   609  				// Break the loop, as otherwise we'd loop
   610  				// forever if d=="." and mdir=="".
   611  				break
   612  			}
   613  			d = parent
   614  		}
   615  	}
   616  
   617  	// Now committed to returning dir (not "").
   618  
   619  	// Are there Go source files in the directory?
   620  	// We don't care about build tags, not even "+build ignore".
   621  	// We're just looking for a plausible directory.
   622  	res := haveGoFilesCache.Do(dir, func() interface{} {
   623  		ok, err := fsys.IsDirWithGoFiles(dir)
   624  		return goFilesEntry{haveGoFiles: ok, err: err}
   625  	}).(goFilesEntry)
   626  
   627  	return dir, res.haveGoFiles, res.err
   628  }
   629  
   630  // fetch downloads the given module (or its replacement)
   631  // and returns its location.
   632  //
   633  // needSum indicates whether the module may be downloaded in readonly mode
   634  // without a go.sum entry. It should only be false for modules fetched
   635  // speculatively (for example, for incompatible version filtering). The sum
   636  // will still be verified normally.
   637  //
   638  // The isLocal return value reports whether the replacement,
   639  // if any, is local to the filesystem.
   640  func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, isLocal bool, err error) {
   641  	if mod == Target {
   642  		return ModRoot(), true, nil
   643  	}
   644  	if r := Replacement(mod); r.Path != "" {
   645  		if r.Version == "" {
   646  			dir = r.Path
   647  			if !filepath.IsAbs(dir) {
   648  				dir = filepath.Join(ModRoot(), dir)
   649  			}
   650  			// Ensure that the replacement directory actually exists:
   651  			// dirInModule does not report errors for missing modules,
   652  			// so if we don't report the error now, later failures will be
   653  			// very mysterious.
   654  			if _, err := fsys.Stat(dir); err != nil {
   655  				if os.IsNotExist(err) {
   656  					// Semantically the module version itself “exists” — we just don't
   657  					// have its source code. Remove the equivalence to os.ErrNotExist,
   658  					// and make the message more concise while we're at it.
   659  					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
   660  				} else {
   661  					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
   662  				}
   663  				return dir, true, module.VersionError(mod, err)
   664  			}
   665  			return dir, true, nil
   666  		}
   667  		mod = r
   668  	}
   669  
   670  	if HasModRoot() && cfg.BuildMod == "readonly" && needSum && !modfetch.HaveSum(mod) {
   671  		return "", false, module.VersionError(mod, &sumMissingError{})
   672  	}
   673  
   674  	dir, err = modfetch.Download(ctx, mod)
   675  	return dir, false, err
   676  }
   677  
   678  type sumMissingError struct {
   679  	suggestion string
   680  }
   681  
   682  func (e *sumMissingError) Error() string {
   683  	return "missing go.sum entry" + e.suggestion
   684  }
   685
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