Fix buildscript diagnostics so diff output is always in unified format

Also make sure diff refers program stdout as 'stdout' rather than '-' in the
test rule diagnostics.

2 files changed, 48 insertions, 12 deletions

diff --git a/libbuild2/script/run.cxx b/libbuild2/script/run.cxx
index b90ba48..8c71b32 100644
--- a/libbuild2/script/run.cxx
+++ b/libbuild2/script/run.cxx
@@ -321,23 +321,44 @@ namespace build2
         path dp ("diff");
         process_path pp (run_search (dp, true));
 
-        cstrings args {pp.recall_string ()};
-
-        // If both files being compared won't be available on failure, then
-        // instruct diff not to print the file paths. It seems that the only
-        // way to achieve this is to abandon the output unified format in the
-        // favor of the minimal output, which normally is still informative
-        // enough for the troubleshooting (contains the difference line
-        // numbers, etc).
-        //
-        if (avail_on_failure (eop, env) || avail_on_failure (op, env))
-          args.push_back ("-u");
+        cstrings args {pp.recall_string (), "-u"};
 
         // Ignore Windows newline fluff if that's what we are running on.
         //
         if (env.host.class_ == "windows")
           args.push_back ("--strip-trailing-cr");
 
+        // Instruct diff not to print the file paths that won't be available
+        // on failure.
+        //
+        // It seems that the only portable way to achieve this is to abandon
+        // the output unified format in the favor of the minimal output.
+        // However, the FreeBSD's, OpenBSD's and GNU's (used on Linux, MacOS,
+        // Windows, and NetBSD) diff utilities support the -L option that
+        // allows to replace the compared file path(s) with custom string(s)
+        // in the utility output. We will use this option for both files if
+        // any of them won't be available on failure (note that we can't
+        // assign a label only for the second file).
+        //
+        // Add the -L option using the file name as its value if it won't be
+        // available on failure and its full path otherwise.
+        //
+        auto add_label = [&args, &env] (const path& p)
+        {
+          const char* s (p.string ().c_str ());
+
+          args.push_back ("-L");
+          args.push_back (avail_on_failure (p, env)
+                          ? s
+                          : path::traits_type::find_leaf (s));
+        };
+
+        if (!avail_on_failure (eop, env) || !avail_on_failure (op, env))
+        {
+          add_label (eop);
+          add_label (op);
+        }
+
         args.push_back (eop.string ().c_str ());
         args.push_back (op.string ().c_str ());
         args.push_back (nullptr);
diff --git a/libbuild2/test/rule.cxx b/libbuild2/test/rule.cxx
index e1006d8..db490d9 100644
--- a/libbuild2/test/rule.cxx
+++ b/libbuild2/test/rule.cxx
@@ -866,7 +866,22 @@ namespace build2
         if (cast<target_triplet> (tt[test_target]).class_ == "windows")
           args.push_back ("--strip-trailing-cr");
 
-        args.push_back (op.string ().c_str ());
+        const char* f (op.string ().c_str ());
+
+        // Note that unmatched program stdout will be referred by diff as '-'
+        // by default. Let's name it as 'stdout' for clarity and consistency
+        // with the buildscript diagnostics.
+        //
+        // Also note that the -L option is not portable but is supported by all
+        // the major implementations (see script/run.cxx for details).
+        //
+        args.push_back ("-L");
+        args.push_back (f);
+
+        args.push_back ("-L");
+        args.push_back ("stdout");
+
+        args.push_back (f);
         args.push_back ("-");
         args.push_back (nullptr);
       }