Test that linux binary depends on glibc 2.18 and no other dynamic symbols (#372)

This regressed twice over the last two months (new floating point
function versions, and accidental dynamic linking against zlib).
We also want to avoid regressions when merging remote index.

The test is able to do a little bit more than we use in the automated
build (the --sym flag is unused, as is unversioned --lib=GLIBC) but they're
pretty useful when experimenting with how to fix things!

We run the test right at the end, because if it fails we want to be able
to download the binary artifact and inspect it.
Unfortunately by the nature of the test we can only run it when we
produce a build, so currently weekly.

.github/workflows/autobuild.yaml

@@ -142,6 +142,9 @@ jobs:
         asset_name: clangd-${{ matrix.config.name }}-${{ github.event.release.tag_name }}.zip
         asset_path: clangd.zip
         asset_content_type: application/zip
+    - name: Check binary compatibility
+      if: matrix.config.name == 'linux'
+      run: .github/worflows/lib_compat_test.py --lib=GLIBC_2.18 "$CLANGD_DIR/bin/clangd"
   # Create the release, and upload the artifacts to it.
   finalize:
     runs-on: ubuntu-latest

.github/workflows/lib_compat_test.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+
+# Verifies a binary uses only dynamic symbols from whitelisted library versions.
+# Prints the disallowed symbols and their versions on failure.
+# Usage: lib_compat_test.py bin/clangd --lib=GLIBC_2.18
+
+import argparse
+import subprocess
+import sys
+
+parser = argparse.ArgumentParser()
+parser.add_argument("binary")
+parser.add_argument(
+    "--lib",
+    action="append",
+    default=[],
+    help="Whitelist a library, e.g. GLIBC_2.18 or GLIBC",
+)
+parser.add_argument(
+    "--sym", action="append", default=[], help="Whitelist a symbol, e.g. crc32"
+)
+args = parser.parse_args()
+
+# Parses GLIBC_2.3 into ("GLIBC", [2,3])
+# Parses GLIBC into ("GLIBC", None)
+def parse_version(version):
+    parts = version.rsplit("_", 1)
+    if len(parts) == 1:
+        return (version, None)
+    try:
+        return (parts[0], [int(p) for p in parts[1].split(".")])
+    except ValueError:
+        return (version, None)
+
+
+lib_versions = dict([parse_version(v) for v in args.lib])
+
+# Determines whether all symbols with version 'lib' are acceptable.
+# A versioned library is name_x.y.z by convention.
+def accept_lib(lib):
+    (lib, ver) = parse_version(lib)
+    if not lib in lib_versions:  # Non-whitelisted library.
+        return False
+    if lib_versions[lib] is None:  # Unrestricted version
+        return True
+    if ver is None:  # Lib has non-numeric version, library restricts version.
+        return False
+    return ver <= lib_versions[lib]
+
+
+# Determines whether an optionally-versioned symbol is acceptable.
+# A versioned symbol is symbol@version as output by nm.
+def accept_symbol(sym):
+    if sym in args.sym:
+        return True
+    split = sym.split("@", 1)
+    return (split[0] in args.sym) or (len(split) == 2 and accept_lib(split[1]))
+
+
+# Run nm to find the undefined symbols, and check whether each is acceptable.
+nm = subprocess.run(
+    ["nm", "-uD", "--with-symbol-version", args.binary],
+    stdout=subprocess.PIPE,
+    text=True,
+)
+nm.check_returncode()
+status = 0
+for line in nm.stdout.splitlines():
+    # line = "       U foo@GLIBC_2.3"
+    parts = line.split()
+    if len(parts) != 2:
+        print("Unparseable nm output: ", line, file=sys.stderr)
+        status = 2
+        continue
+    if parts[0] == "w":  # Weak-undefined symbol, not actually required.
+        continue
+    if not accept_symbol(parts[1]):
+        print(parts[1])
+        status = 1
+if status == 1:
+    print(
+        "Binary depends on disallowed symbols above. Use some combination of:\n"
+        " - relax the whitelist by adding --lib and --sym flags to this test\n"
+        " - force older symbol versions by updating lib_compat.h\n"
+        " - avoid dynamic dependencies by changing CMake configuration\n"
+        " - remove bad dependencies from the code",
+        file=sys.stderr,
+    )
+sys.exit(status)

releases.md

@@ -8,7 +8,8 @@ is being able to cut releases easily whenever we want.
 The releases are just a zip archive containing the `clangd` binary, and the
 clang builtin headers. They should be runnable immediately after extracting the
 archive. The linux binary has `libstdc++` and other dependencies statically
-linked for maximum portability.
+linked for maximum portability, and requires glibc 2.18 (the first version with
+`thread_local` support).
 
 ## Creating a release manually