cmd/go/internal/modfetch: remove error return from Lookup

We generally don't care about errors in resolving a repo if the result
we're looking for is already in the module cache. Moreover, we can
avoid some expense in initializing the repo if all of the methods we
plan to call on it hit in the cache — especially when using
GOPROXY=direct.

This also incidentally fixes a possible (but rare) bug in Download:
we had forgotten to reset the downloaded file in case the Zip method
returned an error after writing a nonzero number of bytes.

For #37438

Change-Id: Ib64f10f763f6d1936536b8e1f7d31ed1b463e955
Reviewed-on: https://go-review.googlesource.com/c/go/+/259158
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

6 files changed, 87 insertions, 87 deletions

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)
-}
-
-// 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)
+	return r.repo().Zip(dst, version)
 }
 
-// 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 unrecoverableErr != nil {
+			return unrecoverableErr
+		}
+		repo := Lookup(proxy, mod.Path)
+		err := repo.Zip(f, mod.Version)
 		if err != nil {
-			return err
+			// 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 repo.Zip(f, mod.Version)
+		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