Correct stat() error check in decomposition library loading

The original code incorrectly used !stat() which would continue the loop
when the library was failed to be loaded (stat returns 0). Change to
continue searching for the library when stat() returns a non-zero.
Otherwise, an error message is reported.

ldms/src/ldmsd/ldmsd_decomp.c

@@ -105,6 +105,7 @@ static ldmsd_decomp_t decomp_get(const char *decomp, ldmsd_req_ctxt_t reqc)
 	char *path = getenv("LDMSD_PLUGIN_LIBPATH");
 	void *d = NULL;
 	char *dlerr;
+	int rc;
 	struct stat st;
 	decomp_rbn_t drbn;
 	ldmsd_decomp_t (*get)(), dc;
@@ -125,33 +126,48 @@ static ldmsd_decomp_t decomp_get(const char *decomp, ldmsd_req_ctxt_t reqc)
 		d = dlopen(library_name, RTLD_NOW);
 		if (d)
 			break;
-		if (!stat(library_name, &st))
+		errno = 0;
+		if ((rc = stat(library_name, &st))) {
+			rc = errno;
+			if (rc == ENOENT) {
+				/*
+				 * We will print a not-found log message
+				 * after we search all paths in pathdir.
+				 *
+				 * Check the next path.
+				 */
+			} else {
+				DECOMP_ERR(reqc, rc, "Failed to load %s. %s\n",
+							library_name, strerror(rc));
+			}
 			continue;
-		dlerr = dlerror();
-		errno = ELIBBAD;
-		DECOMP_ERR(reqc, ELIBBAD, "Bad decomposer '%s': dlerror %s\n",
-			   library_name, dlerr);
-		return NULL;
+		} else {
+			dlerr = dlerror();
+			errno = ELIBBAD;
+			DECOMP_ERR(reqc, ELIBBAD, "Failed to load '%s': dlerror %s\n",
+								library_name, dlerr);
+			return NULL;
+		}
 	}
-
 	if (!d) {
-		dlerr = dlerror();
-		errno = ELIBBAD;
-		DECOMP_ERR(reqc, ELIBBAD, "Failed to load decomposer '%s': "
-				"dlerror %s\n", decomp, dlerr);
+		errno = rc;
+		DECOMP_ERR(reqc, rc, "Cannot load the decomposer %s library.\n", decomp);
 		return NULL;
 	}
 
 	get = dlsym(d, "get");
 	if (!get) {
 		errno = ELIBBAD;
-		DECOMP_ERR(reqc, ELIBBAD, "get() not found in %s\n", libpath);
+		DECOMP_ERR(reqc, ELIBBAD, "The library %s is not a valid "
+					"decomposer plugin.\n", libpath);
 		return NULL;
 	}
 
 	dc = get();
 	if (!dc) {
-		DECOMP_ERR(reqc, errno, "%s:get() error: %d\n", libpath, errno);
+		rc = errno;
+		DECOMP_ERR(reqc, rc, "Failed to get a decomposer '%s' object. %s\n",
+							libpath, strerror(rc));
 		return NULL;
 	}