diff --git a/src/cmd/go/internal/modfetch/cache.go b/src/cmd/go/internal/modfetch/cache.go
index e3074b775e..6eadb026c9 100644
--- a/src/cmd/go/internal/modfetch/cache.go
+++ b/src/cmd/go/internal/modfetch/cache.go
@@ -14,6 +14,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
@@ -155,16 +156,30 @@ func SideLock() (unlock func(), err error) {
 type cachingRepo struct {
 	path  string
 	cache par.Cache // cache for all operations
-	r     Repo
+
+	once     sync.Once
+	initRepo func() (Repo, error)
+	r        Repo
 }
 
-func newCachingRepo(r Repo) *cachingRepo {
+func newCachingRepo(path string, initRepo func() (Repo, error)) *cachingRepo {
 	return &cachingRepo{
-		r:    r,
-		path: r.ModulePath(),
+		path:     path,
+		initRepo: initRepo,
 	}
 }
 
+func (r *cachingRepo) repo() Repo {
+	r.once.Do(func() {
+		var err error
+		r.r, err = r.initRepo()
+		if err != nil {
+			r.r = errRepo{r.path, err}
+		}
+	})
+	return r.r
+}
+
 func (r *cachingRepo) ModulePath() string {
 	return r.path
 }
@@ -175,7 +190,7 @@ func (r *cachingRepo) Versions(prefix string) ([]string, error) {
 		err  error
 	}
 	c := r.cache.Do("versions:"+prefix, func() interface{} {
-		list, err := r.r.Versions(prefix)
+		list, err := r.repo().Versions(prefix)
 		return cached{list, err}
 	}).(cached)
 
@@ -197,7 +212,7 @@ func (r *cachingRepo) Stat(rev string) (*RevInfo, error) {
 			return cachedInfo{info, nil}
 		}
 
-		info, err = r.r.Stat(rev)
+		info, err = r.repo().Stat(rev)
 		if err == nil {
 			// If we resolved, say, 1234abcde to v0.0.0-20180604122334-1234abcdef78,
 			// then save the information under the proper version, for future use.
@@ -224,7 +239,7 @@ func (r *cachingRepo) Stat(rev string) (*RevInfo, error) {
 
 func (r *cachingRepo) Latest() (*RevInfo, error) {
 	c := r.cache.Do("latest:", func() interface{} {
-		info, err := r.r.Latest()
+		info, err := r.repo().Latest()
 
 		// Save info for likely future Stat call.
 		if err == nil {
@@ -258,7 +273,7 @@ func (r *cachingRepo) GoMod(version string) ([]byte, error) {
 			return cached{text, nil}
 		}
 
-		text, err = r.r.GoMod(version)
+		text, err = r.repo().GoMod(version)
 		if err == nil {
 			if err := checkGoMod(r.path, version, text); err != nil {
 				return cached{text, err}
@@ -277,26 +292,11 @@ func (r *cachingRepo) GoMod(version string) ([]byte, error) {
 }
 
 func (r *cachingRepo) Zip(dst io.Writer, version string) error {
-	return r.r.Zip(dst, version)
+	return r.repo().Zip(dst, version)
 }
 
-// Stat is like Lookup(path).Stat(rev) but avoids the
-// repository path resolution in Lookup if the result is
-// already cached on local disk.
-func Stat(proxy, path, rev string) (*RevInfo, error) {
-	_, info, err := readDiskStat(path, rev)
-	if err == nil {
-		return info, nil
-	}
-	repo, err := Lookup(proxy, path)
-	if err != nil {
-		return nil, err
-	}
-	return repo.Stat(rev)
-}
-
-// InfoFile is like Stat but returns the name of the file containing
-// the cached information.
+// InfoFile is like Lookup(path).Stat(version) but returns the name of the file
+// containing the cached information.
 func InfoFile(path, version string) (string, error) {
 	if !semver.IsValid(version) {
 		return "", fmt.Errorf("invalid version %q", version)
@@ -307,10 +307,7 @@ func InfoFile(path, version string) (string, error) {
 	}
 
 	err := TryProxies(func(proxy string) error {
-		repo, err := Lookup(proxy, path)
-		if err == nil {
-			_, err = repo.Stat(version)
-		}
+		_, err := Lookup(proxy, path).Stat(version)
 		return err
 	})
 	if err != nil {
@@ -336,11 +333,7 @@ func GoMod(path, rev string) ([]byte, error) {
 			rev = info.Version
 		} else {
 			err := TryProxies(func(proxy string) error {
-				repo, err := Lookup(proxy, path)
-				if err != nil {
-					return err
-				}
-				info, err := repo.Stat(rev)
+				info, err := Lookup(proxy, path).Stat(rev)
 				if err == nil {
 					rev = info.Version
 				}
@@ -357,11 +350,8 @@ func GoMod(path, rev string) ([]byte, error) {
 		return data, nil
 	}
 
-	err = TryProxies(func(proxy string) error {
-		repo, err := Lookup(proxy, path)
-		if err == nil {
-			data, err = repo.GoMod(rev)
-		}
+	err = TryProxies(func(proxy string) (err error) {
+		data, err = Lookup(proxy, path).GoMod(rev)
 		return err
 	})
 	return data, err
diff --git a/src/cmd/go/internal/modfetch/coderepo_test.go b/src/cmd/go/internal/modfetch/coderepo_test.go
index f69c193b86..28c5e67a28 100644
--- a/src/cmd/go/internal/modfetch/coderepo_test.go
+++ b/src/cmd/go/internal/modfetch/coderepo_test.go
@@ -60,7 +60,6 @@ var altVgotests = map[string]string{
 type codeRepoTest struct {
 	vcs         string
 	path        string
-	lookErr     string
 	mpath       string
 	rev         string
 	err         string
@@ -332,9 +331,9 @@ var codeRepoTests = []codeRepoTest{
 		// package in subdirectory - custom domain
 		// In general we can't reject these definitively in Lookup,
 		// but gopkg.in is special.
-		vcs:     "git",
-		path:    "gopkg.in/yaml.v2/abc",
-		lookErr: "invalid module path \"gopkg.in/yaml.v2/abc\"",
+		vcs:  "git",
+		path: "gopkg.in/yaml.v2/abc",
+		err:  "invalid module path \"gopkg.in/yaml.v2/abc\"",
 	},
 	{
 		// package in subdirectory - github
@@ -440,16 +439,7 @@ func TestCodeRepo(t *testing.T) {
 						testenv.MustHaveExecPath(t, tt.vcs)
 					}
 
-					repo, err := Lookup("direct", tt.path)
-					if tt.lookErr != "" {
-						if err != nil && err.Error() == tt.lookErr {
-							return
-						}
-						t.Errorf("Lookup(%q): %v, want error %q", tt.path, err, tt.lookErr)
-					}
-					if err != nil {
-						t.Fatalf("Lookup(%q): %v", tt.path, err)
-					}
+					repo := Lookup("direct", tt.path)
 
 					if tt.mpath == "" {
 						tt.mpath = tt.path
@@ -685,10 +675,7 @@ func TestCodeRepoVersions(t *testing.T) {
 					testenv.MustHaveExecPath(t, tt.vcs)
 				}
 
-				repo, err := Lookup("direct", tt.path)
-				if err != nil {
-					t.Fatalf("Lookup(%q): %v", tt.path, err)
-				}
+				repo := Lookup("direct", tt.path)
 				list, err := repo.Versions(tt.prefix)
 				if err != nil {
 					t.Fatalf("Versions(%q): %v", tt.prefix, err)
@@ -763,10 +750,7 @@ func TestLatest(t *testing.T) {
 					testenv.MustHaveExecPath(t, tt.vcs)
 				}
 
-				repo, err := Lookup("direct", tt.path)
-				if err != nil {
-					t.Fatalf("Lookup(%q): %v", tt.path, err)
-				}
+				repo := Lookup("direct", tt.path)
 				info, err := repo.Latest()
 				if err != nil {
 					if tt.err != "" {
diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index 1d90002faa..599419977a 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -233,12 +233,28 @@ func downloadZip(ctx context.Context, mod module.Version, zipfile string) (err e
 		}
 	}()
 
+	var unrecoverableErr error
 	err = TryProxies(func(proxy string) error {
-		repo, err := Lookup(proxy, mod.Path)
-		if err != nil {
-			return err
+		if unrecoverableErr != nil {
+			return unrecoverableErr
 		}
-		return repo.Zip(f, mod.Version)
+		repo := Lookup(proxy, mod.Path)
+		err := repo.Zip(f, mod.Version)
+		if err != nil {
+			// Zip may have partially written to f before failing.
+			// (Perhaps the server crashed while sending the file?)
+			// Since we allow fallback on error in some cases, we need to fix up the
+			// file to be empty again for the next attempt.
+			if _, err := f.Seek(0, io.SeekStart); err != nil {
+				unrecoverableErr = err
+				return err
+			}
+			if err := f.Truncate(0); err != nil {
+				unrecoverableErr = err
+				return err
+			}
+		}
+		return err
 	})
 	if err != nil {
 		return err
diff --git a/src/cmd/go/internal/modfetch/repo.go b/src/cmd/go/internal/modfetch/repo.go
index eed4dd4258..4936ec11aa 100644
--- a/src/cmd/go/internal/modfetch/repo.go
+++ b/src/cmd/go/internal/modfetch/repo.go
@@ -188,27 +188,26 @@ type lookupCacheKey struct {
 //
 // A successful return does not guarantee that the module
 // has any defined versions.
-func Lookup(proxy, path string) (Repo, error) {
+func Lookup(proxy, path string) Repo {
 	if traceRepo {
 		defer logCall("Lookup(%q, %q)", proxy, path)()
 	}
 
 	type cached struct {
-		r   Repo
-		err error
+		r Repo
 	}
 	c := lookupCache.Do(lookupCacheKey{proxy, path}, func() interface{} {
-		r, err := lookup(proxy, path)
-		if err == nil {
-			if traceRepo {
+		r := newCachingRepo(path, func() (Repo, error) {
+			r, err := lookup(proxy, path)
+			if err == nil && traceRepo {
 				r = newLoggingRepo(r)
 			}
-			r = newCachingRepo(r)
-		}
-		return cached{r, err}
+			return r, err
+		})
+		return cached{r}
 	}).(cached)
 
-	return c.r, c.err
+	return c.r
 }
 
 // lookup returns the module with the given module path.
@@ -228,7 +227,7 @@ func lookup(proxy, path string) (r Repo, err error) {
 
 	switch proxy {
 	case "off":
-		return nil, errProxyOff
+		return errRepo{path, errProxyOff}, nil
 	case "direct":
 		return lookupDirect(path)
 	case "noproxy":
@@ -407,6 +406,23 @@ func (l *loggingRepo) Zip(dst io.Writer, version string) error {
 	return l.r.Zip(dst, version)
 }
 
+// errRepo is a Repo that returns the same error for all operations.
+//
+// It is useful in conjunction with caching, since cache hits will not attempt
+// the prohibited operations.
+type errRepo struct {
+	modulePath string
+	err        error
+}
+
+func (r errRepo) ModulePath() string { return r.modulePath }
+
+func (r errRepo) Versions(prefix string) (tags []string, err error) { return nil, r.err }
+func (r errRepo) Stat(rev string) (*RevInfo, error)                 { return nil, r.err }
+func (r errRepo) Latest() (*RevInfo, error)                         { return nil, r.err }
+func (r errRepo) GoMod(version string) ([]byte, error)              { return nil, r.err }
+func (r errRepo) Zip(dst io.Writer, version string) error           { return r.err }
+
 // A notExistError is like os.ErrNotExist, but with a custom message
 type notExistError struct {
 	err error
diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
index 24856260d4..65329524f9 100644
--- a/src/cmd/go/internal/modload/mvs.go
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -77,11 +77,7 @@ func versions(ctx context.Context, path string, allowed AllowedFunc) ([]string,
 	// so there's no need for us to add extra caching here.
 	var versions []string
 	err := modfetch.TryProxies(func(proxy string) error {
-		repo, err := modfetch.Lookup(proxy, path)
-		if err != nil {
-			return err
-		}
-		allVersions, err := repo.Versions("")
+		allVersions, err := modfetch.Lookup(proxy, path).Versions("")
 		if err != nil {
 			return err
 		}
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index e75d901ec6..44076fb615 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -229,14 +229,15 @@ func queryProxy(ctx context.Context, proxy, path, query, current string, allowed
 		// If the identifier is not a canonical semver tag — including if it's a
 		// semver tag with a +metadata suffix — then modfetch.Stat will populate
 		// info.Version with a suitable pseudo-version.
-		info, err := modfetch.Stat(proxy, path, query)
+		repo := modfetch.Lookup(proxy, path)
+		info, err := repo.Stat(query)
 		if err != nil {
 			queryErr := err
 			// The full query doesn't correspond to a tag. If it is a semantic version
 			// with a +metadata suffix, see if there is a tag without that suffix:
 			// semantic versioning defines them to be equivalent.
 			if canonicalQuery != "" && query != canonicalQuery {
-				info, err = modfetch.Stat(proxy, path, canonicalQuery)
+				info, err = repo.Stat(canonicalQuery)
 				if err != nil && !errors.Is(err, os.ErrNotExist) {
 					return info, err
 				}
@@ -266,10 +267,7 @@ func queryProxy(ctx context.Context, proxy, path, query, current string, allowed
 	}
 
 	// Load versions and execute query.
-	repo, err := modfetch.Lookup(proxy, path)
-	if err != nil {
-		return nil, err
-	}
+	repo := modfetch.Lookup(proxy, path)
 	versions, err := repo.Versions(prefix)
 	if err != nil {
 		return nil, err