Simplify the building process for end users. (#768)

Use debug.ReadBuildInfo() to generate Version.

Before:

    $ CGO_ENABLED=0 go install ./cmd/goatcounter
    $ goatcounter version
    version=dev; go=go1.23.1; GOOS=linux; GOARCH=amd64; race=false; cgo=false

After:

    $ CGO_ENABLED=0 go install ./cmd/goatcounter
    $ goatcounter version
    version=535cf676f9a3_2024-09-23T18:24:51Z; go=go1.23.1; GOOS=linux; GOARCH=amd64; race=false; cgo=false

After, with a locally modified file:

    $ CGO_ENABLED=0 go install ./cmd/goatcounter
    $ goatcounter version
    version=8ab2fe057181_2024-09-23T18:49:30Z-dev; go=go1.23.1; GOOS=linux; GOARCH=amd64; race=false; cgo=false

Technically this variable should live inside cmd/goatcounter but that
would be a more significant refactoring.

Author: maruel

Diff for: README.md

@@ -132,22 +132,16 @@ You can install from source to $GOBIN (`go env GOBIN`) with:
 
     % git clone --branch=release-2.5 https://github.com/arp242/goatcounter.git
     % cd goatcounter
-    % go build -ldflags="-X zgo.at/goatcounter/v2.Version=$(git log -n1 --format='%h_%cI')" ./cmd/goatcounter
+    % go build ./cmd/goatcounter
 
 Which will produce a `goatcounter` binary in the current directory.
 
-The `-ldflags=[..]` sets the version; this isn't *strictly* required as such,
-but it's recommended as it's used to "bust" the cache for static files and may
-also be useful later when reporting bugs. This can be any string and doesn't
-follow any particular format, you can also set this to the current date or
-`banana` or anything you want really.
-
 To use the latest development version switch to the `master` branch.
 
 To build a fully statically linked binary:
 
     % go build -tags osusergo,netgo,sqlite_omit_load_extension \
-        -ldflags="-X zgo.at/goatcounter/v2.Version=$(git log -n1 --format='%h_%cI') -extldflags=-static" \
+        -extldflags=-static" \
         ./cmd/goatcounter
 
 It's recommended to use the latest release as in the above command. The master
@@ -158,9 +152,7 @@ to surprises.
 You can compile goatcounter without cgo if you're planning to use PostgreSQL and
 don't use SQLite:
 
-    % CGO_ENABLED=0 go build \
-        -ldflags="-X zgo.at/goatcounter.Version=$(git log -n1 --format='%h_%cI')" \
-        ./cmd/goatcounter
+    % CGO_ENABLED=0 go build ./cmd/goatcounter
 
 Functionally it doesn't matter too much, but builds will be a bit easier and
 faster as it won't require a C compiler.

Diff for: context.go

@@ -3,6 +3,7 @@ package goatcounter
 import (
 	"context"
 	"fmt"
+	"runtime/debug"
 	"time"
 
 	"zgo.at/z18n"
@@ -13,10 +14,43 @@ import (
 
 // Version of GoatCounter; set at compile-time with:
 //
-//   -ldflags="-X zgo.at/goatcounter/v2.Version=…"
-
+//	-ldflags="-X zgo.at/goatcounter/v2.Version=…"
 var Version = "dev"
 
+func getCommit() (string, time.Time, bool) {
+	var (
+		rev     string
+		last    time.Time
+		dirty   bool
+		info, _ = debug.ReadBuildInfo()
+	)
+	for _, kv := range info.Settings {
+		switch kv.Key {
+		case "vcs.revision":
+			rev = kv.Value
+		case "vcs.time":
+			last, _ = time.Parse(time.RFC3339, kv.Value)
+		case "vcs.modified":
+			dirty = kv.Value == "true"
+		}
+	}
+	return rev, last, dirty
+}
+
+func init() {
+	if Version == "" || Version == "dev" {
+		// Only calculate the version if not explicitly overridden with:
+		//	-ldflags="-X zgo.at/goatcounter/v2.Version=$tag"
+		// which is done for release builds.
+		if rev, last, dirty := getCommit(); rev != "" {
+			Version = rev[:12] + "_" + last.Format("2006-01-02T15:04:05Z0700")
+			if dirty {
+				Version += "-dev"
+			}
+		}
+	}
+}
+
 var (
 	keyCacheSites      = &struct{ n string }{""}
 	keyCacheUA         = &struct{ n string }{""}