use git log to determine when a file was last updated

broker.go

@@ -6,7 +6,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"time"
 
 	"github.com/motemen/go-wsse"
 	"github.com/x-motemen/blogsync/atom"
@@ -86,11 +85,7 @@ func (b *broker) LocalPath(e *entry) string {
 }
 
 func (b *broker) StoreFresh(e *entry, path string) (bool, error) {
-	var localLastModified time.Time
-	if fi, err := os.Stat(path); err == nil {
-		localLastModified = fi.ModTime()
-	}
-
+	localLastModified, _ := modTime(path)
 	if e.LastModified.After(localLastModified) {
 		logf("fresh", "remote=%s > local=%s", e.LastModified, localLastModified)
 		if err := b.Store(e, path, ""); err != nil {

entry.go

@@ -6,9 +6,11 @@ import (
 	"log"
 	"net/url"
 	"os"
+	"os/exec"
 	"regexp"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/x-motemen/blogsync/atom"
@@ -213,12 +215,11 @@ func entryFromReader(source io.Reader) (*entry, error) {
 			t := time.Now()
 			entry.LastModified = &t
 		} else {
-			fi, err := os.Stat(f.Name())
+			ti, err := modTime(f.Name())
 			if err != nil {
 				return nil, err
 			}
-			t := fi.ModTime()
-			entry.LastModified = &t
+			entry.LastModified = &ti
 		}
 	}
 
@@ -232,3 +233,47 @@ func asEntry(atomEntry *atom.Entry, err error) (*entry, error) {
 
 	return entryFromAtom(atomEntry)
 }
+
+var getGit = sync.OnceValue(func() string {
+	g, err := exec.LookPath("git")
+	if err != nil {
+		return ""
+	}
+	return g
+})
+
+func doCommand(name string, args ...string) (string, error) {
+	cmd := exec.Command(name, args...)
+	cmd.Stderr = os.Stderr
+	bb, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(bb)), nil
+}
+
+func modTime(fpath string) (time.Time, error) {
+	fi, err := os.Stat(fpath)
+	if err != nil {
+		return time.Time{}, err
+	}
+	ti := fi.ModTime()
+
+	if git := getGit(); git != "" {
+		// ref. https://git-scm.com/docs/pretty-formats#Documentation/pretty-formats.txt-emaiem
+		// %aI means "author date, strict ISO 8601 format"
+		timeStr, err := doCommand(git, "log", "-1", "--format=%aI", "--", fpath)
+		if err == nil && timeStr != "" {
+			// check if the file is clean (not modified) on git
+			out, err := doCommand(git, "status", "-s", "--", fpath)
+			// if the output is empty, it means the file has not been modified
+			if err == nil && out == "" {
+				t, err := time.Parse(time.RFC3339, timeStr)
+				if err == nil {
+					ti = t
+				}
+			}
+		}
+	}
+	return ti, nil
+}