Make user/group lookup caching minimally thread-safe

Collect all the cached data into a struct and which is allocated on
per-thread basis, this seems like the path of least trouble in this case
and git diff --stat agrees.

It's worth noting that this also simplifies the caching, we no longer
keep separate name to id and id to name caches (totaling four).
We simply cache one id/name for for user and another for group data.
Also, reset ids to 0 rather than -1, this is far more obviously a safe
value as we have special cases to handle id 0.

rpmugUname() and rpmugGname() are have no users in the current codebase,
so this was developed wrt commit c576d96
where they still are used, to avoid nasty surprises later on if somebody
finds a new case for these.

This all seems like a huge overkill when in 4.19.1 there's exactly one
rpmfiStat() call that unnecessarily invokes an rpmug lookup in a
threaded scenario, but then rpmfiStat() and rpmfilesStat() are public
APIs that people expect to be safe for use within the originating
thread.

Fixes: #2826

lib/rpmug.c

@@ -9,8 +9,16 @@
 #include "rpmug.h"
 #include "debug.h"
 
-static char *pwpath = NULL;
-static char *grppath = NULL;
+struct rpmug_s {
+    char *pwpath;
+    char *grppath;
+    char *lastGname;
+    char *lastUname;
+    uid_t lastUid;
+    gid_t lastGid;
+};
+
+static __thread struct rpmug_s *rpmug = NULL;
 
 static const char *getpath(const char *bn, const char *dfl, char **dest)
 {
@@ -27,12 +35,12 @@ static const char *getpath(const char *bn, const char *dfl, char **dest)
 
 static const char *pwfile(void)
 {
-    return getpath("passwd", "/etc/passwd", &pwpath);
+    return getpath("passwd", "/etc/passwd", &rpmug->pwpath);
 }
 
 static const char *grpfile(void)
 {
-    return getpath("group", "/etc/group", &grppath);
+    return getpath("group", "/etc/group", &rpmug->grppath);
 }
 
 /*
@@ -119,6 +127,12 @@ static int lookup_str(const char *path, long val, int vcol, int rcol,
     return rc;
 }
 
+static void rpmugInit(void)
+{
+    if (rpmug == NULL)
+	rpmug = xcalloc(1, sizeof(*rpmug));
+}
+
 /* 
  * These really ought to use hash tables. I just made the
  * guess that most files would be owned by root or the same person/group
@@ -129,119 +143,103 @@ static int lookup_str(const char *path, long val, int vcol, int rcol,
 
 int rpmugUid(const char * thisUname, uid_t * uid)
 {
-    static char * lastUname = NULL;
-    static uid_t lastUid;
-
-    if (!thisUname) {
-	lastUname = rfree(lastUname);
-	return -1;
-    } else if (rstreq(thisUname, UID_0_USER)) {
+    if (rstreq(thisUname, UID_0_USER)) {
 	*uid = 0;
 	return 0;
     }
 
-    if (lastUname == NULL || !rstreq(thisUname, lastUname)) {
+    rpmugInit();
+
+    if (rpmug->lastUname == NULL || !rstreq(thisUname, rpmug->lastUname)) {
 	long id;
 	if (lookup_num(pwfile(), thisUname, 0, 2, &id))
 	    return -1;
-
-	free(lastUname);
-	lastUname = xstrdup(thisUname);
-	lastUid = id;
+	free(rpmug->lastUname);
+	rpmug->lastUname = xstrdup(thisUname);
+	rpmug->lastUid = id;
     }
 
-    *uid = lastUid;
+    *uid = rpmug->lastUid;
 
     return 0;
 }
 
 int rpmugGid(const char * thisGname, gid_t * gid)
 {
-    static char * lastGname = NULL;
-    static gid_t lastGid;
-
-    if (thisGname == NULL) {
-	lastGname = rfree(lastGname);
-	return -1;
-    } else if (rstreq(thisGname, GID_0_GROUP)) {
+    if (rstreq(thisGname, GID_0_GROUP)) {
 	*gid = 0;
 	return 0;
     }
 
-    if (lastGname == NULL || !rstreq(thisGname, lastGname)) {
+    rpmugInit();
+
+    if (rpmug->lastGname == NULL || !rstreq(thisGname, rpmug->lastGname)) {
 	long id;
 	if (lookup_num(grpfile(), thisGname, 0, 2, &id))
 	    return -1;
-	free(lastGname);
-	lastGname = xstrdup(thisGname);
-	lastGid = id;
+	free(rpmug->lastGname);
+	rpmug->lastGname = xstrdup(thisGname);
+	rpmug->lastGid = id;
     }
 
-    *gid = lastGid;
+    *gid = rpmug->lastGid;
 
     return 0;
 }
 
 const char * rpmugUname(uid_t uid)
 {
-    static uid_t lastUid = (uid_t) -1;
-    static char * lastUname = NULL;
-
-    if (uid == (uid_t) -1) {
-	lastUid = (uid_t) -1;
-	lastUname = rfree(lastUname);
-	return NULL;
-    } else if (uid == (uid_t) 0) {
+    if (uid == (uid_t) 0)
 	return UID_0_USER;
-    } else if (uid == lastUid) {
-	return lastUname;
+
+    rpmugInit();
+
+    if (uid == rpmug->lastUid) {
+	return rpmug->lastUname;
     } else {
 	char *uname = NULL;
 
 	if (lookup_str(pwfile(), uid, 2, 0, &uname))
 	    return NULL;
 
-	lastUid = uid;
-	free(lastUname);
-	lastUname = uname;
+	rpmug->lastUid = uid;
+	free(rpmug->lastUname);
+	rpmug->lastUname = uname;
 
-	return lastUname;
+	return rpmug->lastUname;
     }
 }
 
 const char * rpmugGname(gid_t gid)
 {
-    static gid_t lastGid = (gid_t) -1;
-    static char * lastGname = NULL;
-
-    if (gid == (gid_t) -1) {
-	lastGid = (gid_t) -1;
-	lastGname = rfree(lastGname);
-	return NULL;
-    } else if (gid == (gid_t) 0) {
+    if (gid == (gid_t) 0)
 	return GID_0_GROUP;
-    } else if (gid == lastGid) {
-	return lastGname;
+
+    rpmugInit();
+
+    if (gid == rpmug->lastGid) {
+	return rpmug->lastGname;
     } else {
 	char *gname = NULL;
 
 	if (lookup_str(grpfile(), gid, 2, 0, &gname))
 	    return NULL;
 
-	lastGid = gid;
-	free(lastGname);
-	lastGname = gname;
+	rpmug->lastGid = gid;
+	free(rpmug->lastGname);
+	rpmug->lastGname = gname;
 
-	return lastGname;
+	return rpmug->lastGname;
     }
 }
 
 void rpmugFree(void)
 {
-    rpmugUid(NULL, NULL);
-    rpmugGid(NULL, NULL);
-    rpmugUname(-1);
-    rpmugGname(-1);
-    pwpath = rfree(pwpath);
-    grppath = rfree(grppath);
+    if (rpmug) {
+	free(rpmug->lastUname);
+	free(rpmug->lastGname);
+	free(rpmug->pwpath);
+	free(rpmug->grppath);
+	rpmug = rfree(rpmug);
+    }
 }