cmd/go: add a '-e' flag to 'mod tidy' and 'mod vendor'

This flag, like the -e flag to 'go list', instructs the command to
make a best effort to continue in spite of errors for specific packages.

Fixes #26603

Change-Id: I5ee2f50c71870ae8ef3f9b3e5b045474adcca525
Reviewed-on: https://go-review.googlesource.com/c/go/+/255960
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>

9 files changed, 150 insertions, 21 deletions

diff --git a/doc/go1.16.html b/doc/go1.16.html
index 63ce61b452..09717dac85 100644
--- a/doc/go1.16.html
+++ b/doc/go1.16.html
@@ -66,6 +66,13 @@ Do not send CLs removing the interior tags from such phrases.
   TODO: write and link to tutorial or blog post
 </p>
 
+<p><!-- golang.org/issue/26603 -->
+  The <code>go</code> <code>mod</code> <code>vendor</code>
+  and <code>go</code> <code>mod</code> <code>tidy</code> subcommands now accept
+  the <code>-e</code> flag, which instructs them to proceed despite errors in
+  resolving missing packages.
+</p>
+
 <h4 id="go-test"><code>go</code> <code>test</code></h4>
 
 <p><!-- golang.org/issue/29062 -->
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index a0e93d822e..804ca9e5a5 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1231,7 +1231,7 @@
 //
 // Usage:
 //
-// 	go mod tidy [-v]
+// 	go mod tidy [-e] [-v]
 //
 // Tidy makes sure go.mod matches the source code in the module.
 // It adds any missing modules necessary to build the current module's
@@ -1242,12 +1242,15 @@
 // The -v flag causes tidy to print information about removed modules
 // to standard error.
 //
+// The -e flag causes tidy to attempt to proceed despite errors
+// encountered while loading packages.
+//
 //
 // Make vendored copy of dependencies
 //
 // Usage:
 //
-// 	go mod vendor [-v]
+// 	go mod vendor [-e] [-v]
 //
 // Vendor resets the main module's vendor directory to include all packages
 // needed to build and test all the main module's packages.
@@ -1256,6 +1259,9 @@
 // The -v flag causes vendor to print the names of vendored
 // modules and packages to standard error.
 //
+// The -e flag causes vendor to attempt to proceed despite errors
+// encountered while loading packages.
+//
 //
 // Verify dependencies have expected content
 //
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index df9d9964e6..1cbd7962e7 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -2164,7 +2164,7 @@ func PackagesAndErrors(ctx context.Context, patterns []string) []*Package {
 		loadOpts := modload.PackageOpts{
 			ResolveMissingImports: true,
 			LoadTests:             ModResolveTests,
-			AllowErrors:           true,
+			SilenceErrors:         true,
 		}
 		matches, _ = modload.LoadPackages(ctx, loadOpts, patterns...)
 	} else {
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index 6cba26cc71..fb43e33ec5 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -15,7 +15,7 @@ import (
 )
 
 var cmdTidy = &base.Command{
-	UsageLine: "go mod tidy [-v]",
+	UsageLine: "go mod tidy [-e] [-v]",
 	Short:     "add missing and remove unused modules",
 	Long: `
 Tidy makes sure go.mod matches the source code in the module.
@@ -26,12 +26,18 @@ to go.sum and removes any unnecessary ones.
 
 The -v flag causes tidy to print information about removed modules
 to standard error.
+
+The -e flag causes tidy to attempt to proceed despite errors
+encountered while loading packages.
 	`,
+	Run: runTidy,
 }
 
+var tidyE bool // if true, report errors but proceed anyway.
+
 func init() {
-	cmdTidy.Run = runTidy // break init cycle
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
 	base.AddModCommonFlags(&cmdTidy.Flag)
 }
 
@@ -57,8 +63,9 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 		Tags:                  imports.AnyTags(),
 		ResolveMissingImports: true,
 		LoadTests:             true,
-		AllowErrors:           false, // TODO(#26603): Make this a flag.
+		AllowErrors:           tidyE,
 	}, "all")
+
 	modload.TidyBuildList()
 	modload.TrimGoSum()
 	modload.WriteGoMod()
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index ddc27deb78..1bc4ab3def 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -25,7 +25,7 @@ import (
 )
 
 var cmdVendor = &base.Command{
-	UsageLine: "go mod vendor [-v]",
+	UsageLine: "go mod vendor [-e] [-v]",
 	Short:     "make vendored copy of dependencies",
 	Long: `
 Vendor resets the main module's vendor directory to include all packages
@@ -34,12 +34,18 @@ It does not include test code for vendored packages.
 
 The -v flag causes vendor to print the names of vendored
 modules and packages to standard error.
+
+The -e flag causes vendor to attempt to proceed despite errors
+encountered while loading packages.
 	`,
 	Run: runVendor,
 }
 
+var vendorE bool // if true, report errors but proceed anyway
+
 func init() {
 	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	cmdVendor.Flag.BoolVar(&vendorE, "e", false, "")
 	base.AddModCommonFlags(&cmdVendor.Flag)
 }
 
@@ -54,6 +60,7 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 		Tags:                  imports.AnyTags(),
 		ResolveMissingImports: true,
 		UseVendorAll:          true,
+		AllowErrors:           vendorE,
 	}
 	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 
diff --git a/src/cmd/go/internal/modcmd/why.go b/src/cmd/go/internal/modcmd/why.go
index 5a6d535700..e287c88060 100644
--- a/src/cmd/go/internal/modcmd/why.go
+++ b/src/cmd/go/internal/modcmd/why.go
@@ -66,10 +66,10 @@ func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 	modload.RootMode = modload.NeedRoot
 
 	loadOpts := modload.PackageOpts{
-		Tags:         imports.AnyTags(),
-		LoadTests:    !*whyVendor,
-		AllowErrors:  true,
-		UseVendorAll: *whyVendor,
+		Tags:          imports.AnyTags(),
+		LoadTests:     !*whyVendor,
+		SilenceErrors: true,
+		UseVendorAll:  *whyVendor,
 	}
 
 	if *whyM {
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 371ba8b690..52e3ec84fb 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -406,7 +406,7 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 				Tags:                     imports.AnyTags(),
 				ResolveMissingImports:    true, // dubious; see https://golang.org/issue/32567
 				LoadTests:                *getT,
-				AllowErrors:              true, // Errors may be fixed by subsequent upgrades or downgrades.
+				SilenceErrors:            true, // Errors may be fixed by subsequent upgrades or downgrades.
 				SilenceUnmatchedWarnings: true, // We will warn after iterating below.
 			}
 			matches, _ = modload.LoadPackages(ctx, loadOpts, pkgPatterns...)
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index f268144709..ee5596d16c 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -151,11 +151,14 @@ type PackageOpts struct {
 	// declare 'go 1.16' or higher.
 	UseVendorAll bool
 
-	// AllowErrors indicates that LoadPackages should not log errors in resolving
-	// patterns or imports, and should not terminate the process if such an error
-	// occurs.
+	// AllowErrors indicates that LoadPackages should not terminate the process if
+	// an error occurs.
 	AllowErrors bool
 
+	// SilenceErrors indicates that LoadPackages should not print errors
+	// that occur while loading packages. SilenceErrors implies AllowErrors.
+	SilenceErrors bool
+
 	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
 	// patterns that did not match any packages.
 	SilenceUnmatchedWarnings bool
@@ -263,23 +266,31 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 
 	// One last pass to finalize wildcards.
 	updateMatches(loaded)
-	checkMultiplePaths()
-	WriteGoMod()
 
+	// Report errors, if any.
+	checkMultiplePaths()
 	for _, pkg := range loaded.pkgs {
-		if pkg.err != nil && !opts.AllowErrors {
-			base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+		if pkg.err != nil && !opts.SilenceErrors {
+			if opts.AllowErrors {
+				fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
+			} else {
+				base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+			}
 		}
 		if !pkg.isTest() {
 			loadedPackages = append(loadedPackages, pkg.path)
 		}
 	}
-	if !opts.AllowErrors {
+	if !opts.SilenceErrors {
 		// Also list errors in matching patterns (such as directory permission
 		// errors for wildcard patterns).
 		for _, match := range matches {
 			for _, err := range match.Errs {
-				base.Errorf("%v", err)
+				if opts.AllowErrors {
+					fmt.Fprintf(os.Stderr, "%v\n", err)
+				} else {
+					base.Errorf("%v", err)
+				}
 			}
 		}
 	}
@@ -289,6 +300,8 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 		search.WarnUnmatched(matches)
 	}
 
+	// Success! Update go.mod (if needed) and return the results.
+	WriteGoMod()
 	sort.Strings(loadedPackages)
 	return matches, loadedPackages
 }
diff --git a/src/cmd/go/testdata/script/mod_e.txt b/src/cmd/go/testdata/script/mod_e.txt
new file mode 100644
index 0000000000..3a0d18dabc
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_e.txt
@@ -0,0 +1,89 @@
+cp go.mod go.mod.orig
+
+
+# If a dependency cannot be resolved, 'go mod tidy' fails with an error message
+# explaining the problem and does not update the go.mod file.
+# TODO(bcmills): Ideally, with less redundancy than these error messages!
+
+! go mod tidy
+
+stderr '^example.com/untidy imports\n\texample.net/directnotfound: cannot find module providing package example.net/directnotfound: module example.net/directnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^example.com/untidy imports\n\texample.net/m imports\n\texample.net/indirectnotfound: cannot find module providing package example.net/indirectnotfound: module example.net/indirectnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^example.com/untidy tested by\n\texample.com/untidy.test imports\n\texample.net/directtestnotfound: cannot find module providing package example.net/directtestnotfound: module example.net/directtestnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^example.com/untidy imports\n\texample.net/m tested by\n\texample.net/m.test imports\n\texample.net/indirecttestnotfound: cannot find module providing package example.net/indirecttestnotfound: module example.net/indirecttestnotfound: reading http://.*: 404 Not Found$'
+
+cmp go.mod.orig go.mod
+
+
+# If a dependency cannot be resolved, 'go mod vendor' fails with an error message
+# explaining the problem, does not update the go.mod file, and does not create
+# the vendor directory.
+
+! go mod vendor
+
+stderr '^example.com/untidy imports\n\texample.net/directnotfound: cannot find module providing package example.net/directnotfound: module example.net/directnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^example.com/untidy imports\n\texample.net/m imports\n\texample.net/indirectnotfound: cannot find module providing package example.net/indirectnotfound: module example.net/indirectnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^example.com/untidy tested by\n\texample.com/untidy.test imports\n\texample.net/directtestnotfound: cannot find module providing package example.net/directtestnotfound: module example.net/directtestnotfound: reading http://.*: 404 Not Found$'
+
+! stderr 'indirecttestnotfound'  # Vendor prunes test dependencies.
+
+cmp go.mod.orig go.mod
+! exists vendor
+
+
+# 'go mod tidy' still logs the errors, but succeeds and updates go.mod.
+
+go mod tidy -e
+stderr -count=4 'cannot find module providing package'
+cmp go.mod.final go.mod
+
+
+# 'go mod vendor -e' still logs the errors, but succeeds and updates go.mod.
+
+cp go.mod.orig go.mod
+go mod vendor -e
+stderr -count=3 'cannot find module providing package'
+cmp go.mod.final go.mod
+exists vendor/modules.txt
+exists vendor/example.net/m/m.go
+
+
+-- go.mod --
+module example.com/untidy
+go 1.16
+replace example.net/m v0.1.0 => ./m
+-- go.mod.final --
+module example.com/untidy
+
+go 1.16
+
+replace example.net/m v0.1.0 => ./m
+
+require example.net/m v0.1.0
+-- untidy.go --
+package untidy
+
+import (
+	_ "example.net/m"
+	_ "example.net/directnotfound"
+)
+-- untidy_test.go --
+package untidy_test
+
+import _ "example.net/directtestnotfound"
+-- m/go.mod --
+module example.net/m
+go 1.16
+-- m/m.go --
+package m
+
+import _ "example.net/indirectnotfound"
+-- m/m_test.go --
+package m_test
+
+import _ "example.net/indirecttestnotfound"