internal/task: handle missing commit in greenness

I wanted the greenness check to handle the case where the commit had
aged off the first page of results, and have it pick an arbitrary commit
it could in that case. But I didn't consider that the dashboard doesn't
observe commits instantaneously, and relui can't distinguish the two
cases.

For now, refuse to proceed if the commit isn't on the result page.

For golang/go#48523.

Change-Id: I0a8c9977c1b159f794f2a1d209ef749e606fc422
Reviewed-on: https://go-review.googlesource.com/c/build/+/443436
Auto-Submit: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/task/tagx.go

@@ -471,6 +471,18 @@ func (x *TagXReposTasks) findGreen(ctx *wf.TaskContext, repo TagRepo, commit str
 		}
 	}
 
+	foundCommit := false
+	for _, rev := range repoStatus.Revisions {
+		if rev.Revision == commit {
+			foundCommit = true
+			break
+		}
+	}
+	if !foundCommit {
+		ctx.Printf("commit %v not found on first page of results; too old or too new?", commit)
+		return "", false, nil
+	}
+
 	// x/ repo statuses are:
 	// <x commit> <go commit>
 	//            <go commit>

internal/task/tagx_test.go

@@ -199,6 +199,26 @@ func TestIsGreen(t *testing.T) {
 			},
 			wantGreenRev: "tools-1",
 		},
+		{
+			name: "not green yet",
+			rev:  "tools-1",
+			lines: []revLine{
+				{"master", 3, 1, true},
+				{"release-branch.go1.19", 1, 1, false},
+				{"release-branch.go1.18", 1, 1, true},
+			},
+			wantGreenRev: "",
+		},
+		{
+			name: "commit not registered on dashboard",
+			rev:  "tools-2",
+			lines: []revLine{
+				{"master", 1, 1, true},
+				{"release-branch.go1.19", 1, 1, true},
+				{"release-branch.go1.18", 1, 1, true},
+			},
+			wantGreenRev: "",
+		},
 	}
 	for _, tt := range tests {