Check for NULL returns for both getpwuid and getgrgid

These functions return NULL when the uid or gid does not exist in the
respective database. We were not previously checking for this, so would
crash the process if the entries did not exist.

Fixes #84

NEWS.md

@@ -21,6 +21,8 @@
 
 ## Bugfixes
 
+* Fix crash when a files user or group id did not exist in the respective
+  database (#84, #58)
 * Fix home expansion on systems without readline (#60).
 * Fix propagation of NA values in `path_norm()` (#63).
 

src/file.cc

@@ -192,16 +192,26 @@ List stat_(CharacterVector path) {
     SET_STRING_ELT(VECTOR_ELT(out, 5), i, NA_STRING);
 #else
     passwd* pwd;
-    pwd = getpwuid(st.st_uid);
-    SET_STRING_ELT(VECTOR_ELT(out, 5), i, Rf_mkCharCE(pwd->pw_name, CE_UTF8));
+    if ((pwd = getpwuid(st.st_uid)) != nullptr) {
+      SET_STRING_ELT(VECTOR_ELT(out, 5), i, Rf_mkCharCE(pwd->pw_name, CE_UTF8));
+    } else {
+      char buf[20];
+      sprintf(buf, "%" PRIu64, st.st_uid);
+      SET_STRING_ELT(VECTOR_ELT(out, 5), i, Rf_mkCharCE(buf, CE_UTF8));
+    }
 #endif
 
 #ifdef __WIN32
     SET_STRING_ELT(VECTOR_ELT(out, 6), i, NA_STRING);
 #else
     group* grp;
-    grp = getgrgid(st.st_gid);
-    SET_STRING_ELT(VECTOR_ELT(out, 6), i, Rf_mkCharCE(grp->gr_name, CE_UTF8));
+    if ((grp = getgrgid(st.st_gid)) != nullptr) {
+      SET_STRING_ELT(VECTOR_ELT(out, 6), i, Rf_mkCharCE(grp->gr_name, CE_UTF8));
+    } else {
+      char buf[20];
+      sprintf(buf, "%" PRIu64, st.st_gid);
+      SET_STRING_ELT(VECTOR_ELT(out, 6), i, Rf_mkCharCE(buf, CE_UTF8));
+    }
 #endif
 
     REAL(VECTOR_ELT(out, 7))[i] = st.st_rdev;