bp2build/b: exit early in GENERATE_BAZEL_FILES=1.

This CL fixes a typo in writeFakeNinjaFile to correctly write
a fake out/soong/build.ninja and its depfile. It also modifies bootstrap
phase to *not* run the main soong build phase (which takes more than a
minute) if GENERATE_BAZEL_FILES=1.

This change has the side effect that `GENERATE_BAZEL_FILES=1 m nothing`
no longer generates the real build.ninja, which is fine because one
shouldn't be using GENERATE_BAZEL_FILES=1 for that anyway (or, use
USE_BAZEL_ANALYSIS=1). This change has no effect on mixed builds.

Time on a change to Soong or any Android.bp files:

Before: bp2build_workspace_marker (~20 seconds) + build.ninja (1 min)
After: bp2build_workspace_marker (~20 seconds)

Time on the second of two consecutive `b build //bionic/...`: 2.070s

Test: TH
Test: Soong integration tests
Change-Id: I43720641815994caba97b8d165d7c3fc254cbd06
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index 70c8856..7abb67f 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -175,6 +175,9 @@
 	writeFakeNinjaFile(extraNinjaDeps, configuration.BuildDir())
 }
 
+// doChosenActivity runs Soong for a specific activity, like bp2build, queryview
+// or the actual Soong build for the build.ninja file. Returns the top level
+// output file of the specific activity.
 func doChosenActivity(configuration android.Config, extraNinjaDeps []string) string {
 	bazelConversionRequested := bp2buildMarker != ""
 	mixedModeBuild := configuration.BazelContext.BazelEnabled()
@@ -187,11 +190,7 @@
 		// Run the alternate pipeline of bp2build mutators and singleton to convert
 		// Blueprint to BUILD files before everything else.
 		runBp2Build(configuration, extraNinjaDeps)
-		if bp2buildMarker != "" {
-			return bp2buildMarker
-		} else {
-			return bootstrap.CmdlineArgs.OutFile
-		}
+		return bp2buildMarker
 	}
 
 	ctx := newContext(configuration, prepareBuildActions)
@@ -327,13 +326,13 @@
 
 	ninjaFileName := "build.ninja"
 	ninjaFile := shared.JoinPath(topDir, buildDir, ninjaFileName)
-	ninjaFileD := shared.JoinPath(topDir, buildDir, ninjaFileName)
+	ninjaFileD := shared.JoinPath(topDir, buildDir, ninjaFileName+".d")
 	// A workaround to create the 'nothing' ninja target so `m nothing` works,
 	// since bp2build runs without Kati, and the 'nothing' target is declared in
 	// a Makefile.
 	ioutil.WriteFile(ninjaFile, []byte("build nothing: phony\n  phony_output = true\n"), 0666)
 	ioutil.WriteFile(ninjaFileD,
-		[]byte(fmt.Sprintf("%s: \\\n %s\n", ninjaFileName, extraNinjaDepsString)),
+		[]byte(fmt.Sprintf("%s: \\\n %s\n", ninjaFile, extraNinjaDepsString)),
 		0666)
 }
 
@@ -520,9 +519,14 @@
 		os.Exit(1)
 	}
 
-	if bp2buildMarker != "" {
-		touch(shared.JoinPath(topDir, bp2buildMarker))
-	} else {
-		writeFakeNinjaFile(extraNinjaDeps, codegenContext.Config().BuildDir())
-	}
+	// Create an empty bp2build marker file.
+	touch(shared.JoinPath(topDir, bp2buildMarker))
+
+	// bp2build *always* writes a fake Ninja file containing just the nothing
+	// phony target if it ever re-runs. This allows bp2build to exit early with
+	// GENERATE_BAZEL_FILES=1 m nothing.
+	//
+	// If bp2build is invoked as part of an integrated mixed build, the fake
+	// build.ninja file will be rewritten later into the real file anyway.
+	writeFakeNinjaFile(extraNinjaDeps, codegenContext.Config().BuildDir())
 }