init: Check DecodeUid() result and use error string

Check the result of DecodeUid() and return failure when uids/gids are
unable to be decoded.

Also, use an error string instead of logging directly such that more
context can be added when decoding fails.

Bug: 38038887
Test: Boot bullhead
Test: Init unit tests
Change-Id: I84c11aa5a8041bf5d2f754ee9af748344b789b37
diff --git a/init/util.cpp b/init/util.cpp
index 87b8d9d..4df0c25 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -48,39 +48,33 @@
 #endif
 
 using android::base::boot_clock;
+using namespace std::literals::string_literals;
 
-static unsigned int do_decode_uid(const char *s)
-{
-    unsigned int v;
+// DecodeUid() - decodes and returns the given string, which can be either the
+// numeric or name representation, into the integer uid or gid. Returns
+// UINT_MAX on error.
+bool DecodeUid(const std::string& name, uid_t* uid, std::string* err) {
+    *uid = UINT_MAX;
+    *err = "";
 
-    if (!s || *s == '\0')
-        return UINT_MAX;
-
-    if (isalpha(s[0])) {
-        struct passwd* pwd = getpwnam(s);
-        if (!pwd)
-            return UINT_MAX;
-        return pwd->pw_uid;
+    if (isalpha(name[0])) {
+        passwd* pwd = getpwnam(name.c_str());
+        if (!pwd) {
+            *err = "getpwnam failed: "s + strerror(errno);
+            return false;
+        }
+        *uid = pwd->pw_uid;
+        return true;
     }
 
     errno = 0;
-    v = (unsigned int) strtoul(s, 0, 0);
-    if (errno)
-        return UINT_MAX;
-    return v;
-}
-
-/*
- * decode_uid - decodes and returns the given string, which can be either the
- * numeric or name representation, into the integer uid or gid. Returns
- * UINT_MAX on error.
- */
-unsigned int decode_uid(const char *s) {
-    unsigned int v = do_decode_uid(s);
-    if (v == UINT_MAX) {
-        LOG(ERROR) << "decode_uid: Unable to find UID for '" << s << "'; returning UINT_MAX";
+    uid_t result = static_cast<uid_t>(strtoul(name.c_str(), 0, 0));
+    if (errno) {
+        *err = "strtoul failed: "s + strerror(errno);
+        return false;
     }
-    return v;
+    *uid = result;
+    return true;
 }
 
 /*