Run libtool without symlinking if possible

tools/objc/BUILD

@@ -16,6 +16,14 @@ sh_binary(
     data = [
         ":make_hashed_objlist.py",
         ":xcrunwrapper",
+        ":libtool_check_unique",
+    ],
+)
+
+cc_binary(
+    name = "libtool_check_unique",
+    srcs = [
+        "libtool_check_unique.cc",
     ],
 )
 

tools/objc/libtool.sh

@@ -34,6 +34,15 @@ fi
 
 WRAPPER="${MY_LOCATION}/xcrunwrapper.sh"
 
+# Ensure 0 timestamping for hermetic results.
+export ZERO_AR_DATE=1
+
+if $(/usr/bin/dirname "$0")/libtool_check_unique "$@"; then
+  # If there are no duplicate .o basenames, libtool can be invoked with the original arguments
+  "${WRAPPER}" libtool "$@"
+  exit
+fi
+
 TEMPDIR="$(mktemp -d "${TMPDIR:-/tmp}/libtool.XXXXXXXX")"
 trap "rm -rf \"${TEMPDIR}\"" EXIT
 
@@ -111,11 +120,4 @@ while [[ $# -gt 0 ]]; do
    esac
 done
 
-# Ensure 0 timestamping for hermetic results.
-export ZERO_AR_DATE=1
-
 "${WRAPPER}" libtool "${ARGS[@]}"
-
-# Prevents a pre-Xcode-8 bug in which passing zero-date archive files to ld
-# would cause ld to error.
-touch "$OUTPUTFILE"

tools/objc/libtool_check_unique.cc

@@ -0,0 +1,57 @@
+#include <iostream>
+#include <regex>
+#include <fstream>
+#include <unordered_set>
+
+using namespace std;
+
+string getBasename(const string &path) {
+    // Assumes we're on an OS with "/" as the path separator
+    auto idx = path.find_last_of("/");
+    if (idx == string::npos) {
+        return path;
+    }
+    return path.substr(idx + 1);
+}
+
+// Returns 0 if there are no duplicate basenames in the object files (both via -filelist as well as shell args),
+// 1 otherwise
+int main(int argc, const char * argv[]) {
+    unordered_set<string> basenames;
+    const regex libRegex = regex(".*\\.a$");
+    const regex noArgFlags = regex("-static|-s|-a|-c|-L|-T|-no_warning_for_no_symbols");
+    const regex singleArgFlags = regex("-arch_only|-syslibroot|-o");
+    // Set i to 1 to skip executable path
+    for (int i = 1; argv[i] != nullptr; i++) {
+        string arg = argv[i];
+        if (arg == "-filelist") {
+            ifstream list(argv[i + 1]);
+            for (string line; getline(list, line); ) {
+                const string basename = getBasename(line);
+                const auto pair = basenames.insert(basename);
+                if (!pair.second) {
+                    return 1;
+                }
+            }
+            list.close();
+            i++;
+        } else if (regex_match(arg, noArgFlags)) {
+            // No arg flags
+        } else if (regex_match(arg, singleArgFlags)) {
+            // Single-arg flags
+            i++;
+        } else if (arg[0] == '-') {
+            return 1;
+            // Unrecognized flag, let the wrapper deal with it
+        } else if (regex_match(arg, libRegex)) {
+            // Archive inputs can remain untouched, as they come from other targets.
+        } else {
+            const string basename = getBasename(arg);
+            const auto pair = basenames.insert(basename);
+            if (!pair.second) {
+                return 1;
+            }
+        }
+    }
+    return 0;
+}