[symbolizer] Empty string is not an error (#92660)

After commit 1792852f86dc7 ([symbolizer] Change reaction on invalid
input) llvm-symbolizer issues an error on malformed command instead of
echoing it to the standard output, as in previous versions. It turns out
this behavior broke a use case when echoing was used to check if
llvm-symbolizer is working (https://github.com/llvm/llvm-project/commit/1792852f86dc75efa1f44d46b1a0daf386d64afa#commitcomment-142161925).

With this change an empty line as input is not considered as an error
anymore and does not produce any output on stderr. llvm-symbolizer still
respond on empty line with line not found, this is consistent with GNU
addr2line.

---------

Co-authored-by: James Henderson <46713263+jh7370@users.noreply.github.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp b/llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp
index b98bdbc..6d7953f 100644
--- a/llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp
+++ b/llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp
@@ -337,6 +337,14 @@ static void symbolizeInput(const opt::InputArgList &Args,
   object::BuildID BuildID(IncomingBuildID.begin(), IncomingBuildID.end());
   uint64_t Offset = 0;
   StringRef Symbol;
+
+  // An empty input string may be used to check if the process is alive and
+  // responding to input. Do not emit a message on stderr in this case but
+  // respond on stdout.
+  if (InputString.empty()) {
+    printUnknownLineInfo(ModuleName, Printer);
+    return;
+  }
   if (Error E = parseCommand(Args.getLastArgValue(OPT_obj_EQ), IsAddr2Line,
                              StringRef(InputString), Cmd, ModuleName, BuildID,
                              Symbol, Offset)) {