cmd/go: disallow go.sum updates in -mod=readonly

When running go build with the flag -mod=readonly, it fails the build if
go.sum files requires updating. This ensures that CI/CD systems get a
complete go.sum file so that they'd never hit a notary,
assuming the CI/CD system passes the above flag.
I am not familiar with the entire codebase but I assume goSum.dirty
will always be true if go.sum has any missing lines.

Fixes #30667

Change-Id: I767d3b594055d8c10048f4c68e6687c94bb0545c
Reviewed-on: https://go-review.googlesource.com/c/go/+/166237Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modfetch/fetch.go

@@ -509,6 +509,9 @@ func WriteGoSum() {
 		// Don't bother opening the go.sum file if we don't have anything to add.
 		return
 	}
+	if cfg.BuildMod == "readonly" {
+		base.Fatalf("go: updates to go.sum needed, disabled by -mod=readonly")
+	}
 
 	// We want to avoid races between creating the lockfile and deleting it, but
 	// we also don't want to leave a permanent lockfile in the user's repository.

src/cmd/go/internal/modload/init.go

@@ -665,18 +665,21 @@ func WriteGoMod() {
 		base.Fatalf("go: %v", err)
 	}
 
+	dirty := !bytes.Equal(new, modFileData)
+	if dirty && cfg.BuildMod == "readonly" {
+		// If we're about to fail due to -mod=readonly,
+		// prefer to report a dirty go.mod over a dirty go.sum
+		base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly")
+	}
 	// Always update go.sum, even if we didn't change go.mod: we may have
 	// downloaded modules that we didn't have before.
 	modfetch.WriteGoSum()
 
-	if bytes.Equal(new, modFileData) {
+	if !dirty {
 		// We don't need to modify go.mod from what we read previously.
 		// Ignore any intervening edits.
 		return
 	}
-	if cfg.BuildMod == "readonly" {
-		base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly")
-	}
 
 	unlock := modfetch.SideLock()
 	defer unlock()

src/cmd/go/testdata/script/mod_file_proxy.txt

@@ -23,3 +23,10 @@ require rsc.io/quote v1.5.1
 -- $WORK/x/x.go --
 package x
 import _ "rsc.io/quote"
+-- $WORK/x/go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.1 h1:ZE3OgnVGrhXtFkGw90HwW992ZRqcdli/33DLqEYsoxA=
+rsc.io/quote v1.5.1/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
\ No newline at end of file

src/cmd/go/testdata/script/sum_readonly.txt

+# Test that go.sum does not get updated when -mod=readonly flag is set
+env GO111MODULE=on
+
+go get rsc.io/quote
+go mod tidy
+
+# go.sum != dirty; -mod=readonly
+go build -mod=readonly
+
+# dirty up go.sum by removing it.
+rm go.sum
+
+# go.sum == dirty; -mod=readonly
+! go build -mod=readonly
+
+stderr 'go: updates to go.sum needed, disabled by -mod=readonly'
+
+-- go.mod --
+module m
+
+-- main.go --
+
+package main
+
+import "rsc.io/quote"
+
+func main() {
+    println(quote.Hello())
+}
\ No newline at end of file