Start proper approach to determining how build_always targets should work by writing a proper test case illustrating how it should behave.

test cases/common/72 build always/main.c

@@ -0,0 +1,7 @@
+#include<stdio.h>
+#include"version.h"
+
+int main(int argc, char **argv) {
+    printf("Version is %s.\n", version_string);
+    return 0;
+}

test cases/common/72 build always/meson.build

@@ -0,0 +1,15 @@
+project('run always', 'c')
+
+version = '1.0.0'
+
+vgen = find_program('version_gen.py')
+
+version_src = custom_target('Version string',
+input : 'version.c.in',
+output : 'version.c',
+command : [vgen, '@INPUT@', '@OUTPUT@', version],
+build_always : true,
+)
+
+executable('versionprinter', 'main.c', version_src,
+include_directories : include_directories('.'))

test cases/common/72 build always/version.c.in

@@ -0,0 +1,3 @@
+#include"version.h"
+
+const char *version_string = "@VERSION@";

test cases/common/72 build always/version.h

@@ -0,0 +1,3 @@
+#pragma once
+
+const char *version_string;

test cases/common/72 build always/version_gen.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python3
+
+import sys, os, subprocess
+
+def generate(infile, outfile, fallback):
+    workdir = os.path.split(infile)[0]
+    if workdir == '':
+        workdir = '.'
+    p = subprocess.Popen(['git', 'describe'], cwd=workdir, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    (stdo, _) = p.communicate()
+    # If we are working off an extracted tarball, git version number is not available.
+    if p.returncode == 0:
+        version = stdo.decode().strip()
+    else:
+        version = fallback
+    newdata = open(infile).read().replace('@VERSION@', version)
+    try:
+        olddata = open(outfile).read()
+        if olddata == newdata:
+            return
+    except Exception:
+        pass
+    open(outfile, 'w').write(newdata)
+
+if __name__ == '__main__':
+    infile = sys.argv[1]
+    outfile = sys.argv[2]
+    fallback = sys.argv[3]
+    generate(infile, outfile, fallback)