chg: make is possible to call by default an hg binary located next to chg

chg: make is possible to call by default an hg binary located next to chg

When a single version of hg is in use and it's in the PATH, using chg
is just a matter of calling chg.

But when there are multiple installations of hg+chg around, and hg is
referred to with an absolute path, using chg is more annoying because
it requires both changing the invocation to hg to use chg, but also
setting CHGHG.

Currently, we set HGPATH when we build chg to remove the need to set
CHGHG in the previous paragraph. But that means chg now hardcodes its
installation path, which makes the installation not relocatable. Hence
this proposal to make chg find ./hg relative to itself (as opposed to
CHGHG=./hg which find hg relative to cwd).

This only works on linux as written, but since it's opt-in, it sounds
fine.

Tested by hand, as I'm not sure how else to test this.

Differential Revision: https://phab.mercurial-scm.org/D9006

--- a/contrib/chg/Makefile	Wed Sep 23 14:44:21 2020 +0200
+++ b/contrib/chg/Makefile	Thu Sep 03 11:07:47 2020 -0400
@@ -8,6 +8,9 @@
 ifdef HGPATH
 override CPPFLAGS += -DHGPATH=\"$(HGPATH)\"
 endif
+ifdef HGPATHREL
+override CPPFLAGS += -DHGPATHREL=\"$(HGPATHREL)\"
+endif
 
 DESTDIR =
 PREFIX = /usr/local

--- a/contrib/chg/README	Wed Sep 23 14:44:21 2020 +0200
+++ b/contrib/chg/README	Thu Sep 03 11:07:47 2020 -0400
@@ -30,3 +30,11 @@
  * CHGSOCKNAME specifies the socket path of the background cmdserver.
  * CHGTIMEOUT specifies how many seconds chg will wait before giving up
    connecting to a cmdserver. If it is 0, chg will wait forever. Default: 60
+
+Build environment variables:
+
+ * HGPATH: the path to the hg executable to call when CHGHG and HG are not set,
+   instead of "hg"
+ * HGPATHREL=1: when CHGHG and HG are not set, the hg executable will be ./hg
+   relative to the chg executable. Only works on linux, falls back to "hg"
+   otherwise.

--- a/contrib/chg/chg.c	Wed Sep 23 14:44:21 2020 +0200
+++ b/contrib/chg/chg.c	Thu Sep 03 11:07:47 2020 -0400
@@ -184,13 +184,46 @@
 		abortmsg("too long TMPDIR or CHGSOCKNAME (r = %d)", r);
 }
 
+/* If the current program is, say, /a/b/c/chg, returns /a/b/c/hg. */
+static char *getrelhgcmd(void)
+{
+	ssize_t n;
+	char *res, *slash;
+	int maxsize = 4096;
+	res = malloc(maxsize);
+	if (res == NULL)
+		goto cleanup;
+	n = readlink("/proc/self/exe", res, maxsize);
+	if (n < 0 || n >= maxsize)
+		goto cleanup;
+	res[n] = '\0';
+	slash = strrchr(res, '/');
+	if (slash == NULL)
+		goto cleanup;
+	/* 4 is strlen("/hg") + nul byte */
+	if (slash + 4 >= res + maxsize)
+		goto cleanup;
+	memcpy(slash, "/hg", 4);
+	return res;
+cleanup:
+	free(res);
+	return NULL;
+}
+
 static const char *gethgcmd(void)
 {
 	static const char *hgcmd = NULL;
+#ifdef HGPATHREL
+	int tryrelhgcmd = 1;
+#else
+	int tryrelhgcmd = 0;
+#endif
 	if (!hgcmd) {
 		hgcmd = getenv("CHGHG");
 		if (!hgcmd || hgcmd[0] == '\0')
 			hgcmd = getenv("HG");
+		if (tryrelhgcmd && (!hgcmd || hgcmd[0] == '\0'))
+			hgcmd = getrelhgcmd();
 		if (!hgcmd || hgcmd[0] == '\0')
 #ifdef HGPATH
 			hgcmd = (HGPATH);