Implement getpwnam_r(3) and getpwuid_r(3).

These functions were already declared in <pwd.h>, but hadn't been implemented
yet.

git cherry-pick --no-commit 081504af74826bad7035669ad34d457b4b439e8f

Change-Id: I316acf4cffb9f2c6788e8e342aa620f9a00886d5

libc/bionic/stubs.c

@@ -37,6 +37,78 @@
 #include <errno.h>
 #include <ctype.h>
 
+static int do_getpw_r(int by_name, const char* name, uid_t uid,
+        struct passwd* dst, char* buf, size_t byte_count, struct passwd** result)
+{
+    /*
+     * getpwnam_r and getpwuid_r don't modify errno, but library calls we
+     *  make might.
+     */
+    int old_errno = errno;
+    int rc = 0;
+    *result = NULL;
+
+    const struct passwd* src = by_name ? getpwnam(name) : getpwuid(uid);
+
+    /*
+     * POSIX allows failure to find a match to be considered a non-error.
+     * Reporting success (0) but with *result NULL is glibc's behavior.
+     */
+    if (src == NULL) {
+        rc = (errno == ENOENT) ? 0 : errno;
+        goto failure;
+    }
+
+    /*
+     * Work out where our strings will go in 'buf', and whether we've got
+     * enough space.
+     */
+    size_t required_byte_count = 0;
+    dst->pw_name = buf;
+    required_byte_count += strlen(src->pw_name) + 1;
+    dst->pw_dir = buf + required_byte_count;
+    required_byte_count += strlen(src->pw_dir) + 1;
+    dst->pw_shell = buf + required_byte_count;
+    required_byte_count += strlen(src->pw_shell) + 1;
+    if (byte_count < required_byte_count) {
+        rc = ERANGE;
+        goto failure;
+    }
+
+    /* Copy the strings. */
+    snprintf(buf, byte_count, "%s%c%s%c%s",
+            src->pw_name, 0, src->pw_dir, 0, src->pw_shell);
+
+    /*
+     * pw_passwd is non-POSIX and unused (always NULL) in bionic.
+     * pw_gecos is non-POSIX and missing in bionic.
+     */
+    dst->pw_passwd = NULL;
+
+    /* Copy the integral fields. */
+    dst->pw_gid = src->pw_gid;
+    dst->pw_uid = src->pw_uid;
+
+success:
+    rc = 0;
+    *result = dst;
+failure:
+    errno = old_errno;
+    return rc;
+}
+
+int getpwnam_r(const char* name, struct passwd* pwd,
+        char* buf, size_t byte_count, struct passwd** result)
+{
+    return do_getpw_r(1, name, -1, pwd, buf, byte_count, result);
+}
+
+int getpwuid_r(uid_t uid, struct passwd* pwd,
+        char* buf, size_t byte_count, struct passwd** result)
+{
+    return do_getpw_r(0, NULL, uid, pwd, buf, byte_count, result);
+}
+
 /** Thread-specific state for the stubs functions
  **/
 
@@ -400,4 +472,3 @@ void endusershell(void)
 {
     fprintf(stderr, "FIX ME! implement %s() %s:%d\n", __FUNCTION__, __FILE__, __LINE__);
 }
-