.dockerignore, internal/build: Read git information directly from file (#15458)

* .dockerignore, internal/build: Read git information directly from file

This commit changes the way of retrieving git commit and branch for build
environment from running git command to reading git files directly.

This commit also adds required git files into Docker build context.

fixes: #15346

* .dockerignore: workaround for including some files in .git

.dockerignore

 **/.git
+/.git
+!/.git/HEAD
+!/.git/refs/heads
 **/*_test.go
 
 build/_workspace

internal/build/env.go

@@ -82,18 +82,21 @@ func Env() Environment {
 // LocalEnv returns build environment metadata gathered from git.
 func LocalEnv() Environment {
 	env := applyEnvFlags(Environment{Name: "local", Repo: "ethereum/go-ethereum"})
-	if _, err := os.Stat(".git"); err != nil {
+	head := ReadGitFile("HEAD")
+	if splits := strings.Split(head, " "); len(splits) == 2 {
+		head = splits[1]
+	} else {
 		return env
 	}
 	if env.Commit == "" {
-		env.Commit = RunGit("rev-parse", "HEAD")
+		env.Commit = ReadGitFile(head)
 	}
 	if env.Branch == "" {
-		if b := RunGit("rev-parse", "--abbrev-ref", "HEAD"); b != "HEAD" {
-			env.Branch = b
+		if head != "HEAD" {
+			env.Branch = strings.TrimLeft(head, "refs/heads/")
 		}
 	}
-	if env.Tag == "" {
+	if info, err := os.Stat(".git/objects"); err == nil && info.IsDir() && env.Tag == "" {
 		env.Tag = firstLine(RunGit("tag", "-l", "--points-at", "HEAD"))
 	}
 	return env

internal/build/util.go

@@ -25,6 +25,7 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -88,6 +89,15 @@ func RunGit(args ...string) string {
 	return strings.TrimSpace(stdout.String())
 }
 
+// ReadGitFile returns content of file in .git directory.
+func ReadGitFile(file string) string {
+	content, err := ioutil.ReadFile(path.Join(".git", file))
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(content))
+}
+
 // Render renders the given template file into outputFile.
 func Render(templateFile, outputFile string, outputPerm os.FileMode, x interface{}) {
 	tpl := template.Must(template.ParseFiles(templateFile))