From f087764abcba7e041fb5c8c59f6122861b00ddba Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9my=20Oudompheng?= <oudomphe@phare.normalesup.org>
Date: Thu, 9 Aug 2012 23:22:51 +0200
Subject: [PATCH] go/build: correct shouldBuild bug reading whole contents of
 file.

It was caused by bytes.TrimSpace being able to return a nil
slice.

Fixes #3914.

R=golang-dev, r
CC=golang-dev, remy
https://golang.org/cl/6458091
---
 src/pkg/go/build/build.go      |  2 +-
 src/pkg/go/build/build_test.go | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/pkg/go/build/build.go b/src/pkg/go/build/build.go
index c8a0808efd4dd..a65ff437ab347 100644
--- a/src/pkg/go/build/build.go
+++ b/src/pkg/go/build/build.go
@@ -689,7 +689,7 @@ func (ctxt *Context) shouldBuild(content []byte) bool {
 		}
 		line = bytes.TrimSpace(line)
 		if len(line) == 0 { // Blank line
-			end = cap(content) - cap(line) // &line[0] - &content[0]
+			end = len(content) - len(p)
 			continue
 		}
 		if !bytes.HasPrefix(line, slashslash) { // Not comment line
diff --git a/src/pkg/go/build/build_test.go b/src/pkg/go/build/build_test.go
index 560ebad5c97a9..caa4f26f332e6 100644
--- a/src/pkg/go/build/build_test.go
+++ b/src/pkg/go/build/build_test.go
@@ -75,3 +75,32 @@ func TestLocalDirectory(t *testing.T) {
 		t.Fatalf("ImportPath=%q, want %q", p.ImportPath, "go/build")
 	}
 }
+
+func TestShouldBuild(t *testing.T) {
+	const file1 = "// +build tag1\n\n" +
+		"package main\n"
+
+	const file2 = "// +build cgo\n\n" +
+		"// This package implements parsing of tags like\n" +
+		"// +build tag1\n" +
+		"package build"
+
+	const file3 = "// Copyright The Go Authors.\n\n" +
+		"package build\n\n" +
+		"// shouldBuild checks tags given by lines of the form\n" +
+		"// +build tag\n" +
+		"func shouldBuild(content []byte)\n"
+
+	ctx := &Context{BuildTags: []string{"tag1"}}
+	if !ctx.shouldBuild([]byte(file1)) {
+		t.Errorf("should not build file1, expected the contrary")
+	}
+	if ctx.shouldBuild([]byte(file2)) {
+		t.Errorf("should build file2, expected the contrary")
+	}
+
+	ctx = &Context{BuildTags: nil}
+	if !ctx.shouldBuild([]byte(file3)) {
+		t.Errorf("should not build file3, expected the contrary")
+	}
+}