unified: Harden the external Swift parser integration

Two fixes prompted by review of the swift-syntax switch-over.

Parser resolution (`parse.rs`): `parse_bin` now resolves the
`swift-syntax-parse` executable in priority order — the
`CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE` override, then a copy next
to the extractor executable (as a shipped extractor pack lays it out:
`tools/<platform>/{extractor,swift-syntax-parse}`), then a
bare `PATH` lookup. This lets a packaged extractor find its parser with
no environment setup. (Bundling the binary into the pack, together with
its Swift runtime, is a separate follow-up.)

Corpus test guard (`corpus_tests.rs`): `parser_available` previously
treated *any* parser error as "unavailable" and skipped the entire
corpus suite, so a parser that was present but crashed or emitted
invalid JSON would silently skip the exact regressions the suite exists
to catch. It now uses the new `binary_available`, which reports whether
the *executable* can be launched (false only when it cannot be found,
e.g. no Swift toolchain); a launchable-but-failing parser makes the
suite run and fail.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

unified/extractor/src/languages/swift/parse.rs

@@ -16,9 +16,12 @@ use codeql_extractor::extractor::ParsedTree;
 use super::swift_adapter;
 
 /// Environment variable naming the `swift-syntax-parse` executable. When unset,
-/// `swift-syntax-parse` is looked up on `PATH`.
+/// the parser is resolved next to the extractor executable, then on `PATH`.
 const PARSE_BIN_ENV: &str = "CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE";
 
+/// Base name of the `swift-syntax-parse` executable as shipped / looked up.
+const PARSE_BIN_NAME: &str = "swift-syntax-parse";
+
 /// Parse Swift `source` into a [`ParsedTree`] (a raw `yeast::Ast` plus
 /// side-channel `extra` tokens), ready to be desugared via `run_from_ast`.
 pub fn parse(source: &[u8]) -> Result<ParsedTree, String> {
@@ -33,9 +36,54 @@ pub fn parse(source: &[u8]) -> Result<ParsedTree, String> {
     })
 }
 
-/// The `swift-syntax-parse` executable to invoke.
+/// The `swift-syntax-parse` executable to invoke, resolved in priority order:
+///
+/// 1. the `CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE` override, if set;
+/// 2. a copy shipped next to the extractor executable — this is how the CodeQL
+///    extractor pack lays it out (`tools/<platform>/{extractor,
+///    swift-syntax-parse}`), so a packaged extractor is self-contained with no
+///    environment setup;
+/// 3. a bare `swift-syntax-parse`, looked up on `PATH`.
 fn parse_bin() -> String {
-    std::env::var(PARSE_BIN_ENV).unwrap_or_else(|_| "swift-syntax-parse".to_string())
+    if let Ok(bin) = std::env::var(PARSE_BIN_ENV) {
+        if !bin.is_empty() {
+            return bin;
+        }
+    }
+    if let Ok(exe) = std::env::current_exe() {
+        if let Some(sibling) = exe.parent().map(|dir| dir.join(PARSE_BIN_NAME)) {
+            if sibling.is_file() {
+                return sibling.to_string_lossy().into_owned();
+            }
+        }
+    }
+    PARSE_BIN_NAME.to_string()
+}
+
+/// Whether the `swift-syntax-parse` executable can be launched at all.
+///
+/// This reports availability of the *executable*, deliberately not whether
+/// parsing succeeds: a binary that launches but then crashes or emits invalid
+/// JSON is still "available", so callers run and surface the failure rather
+/// than silently skipping. Only a genuinely missing/unlaunchable binary (e.g.
+/// no Swift toolchain is installed) reports `false`.
+pub fn binary_available() -> bool {
+    match Command::new(parse_bin())
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .spawn()
+    {
+        Ok(mut child) => {
+            let _ = child.wait();
+            true
+        }
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => false,
+        // Any other spawn failure (e.g. a permissions problem) is a genuine
+        // issue worth surfacing, so treat the parser as available and let the
+        // caller fail rather than masking it as "unavailable".
+        Err(_) => true,
+    }
 }
 
 /// Run the external parser, feeding `source` on stdin and returning its JSON

unified/extractor/tests/corpus_tests.rs

@@ -20,12 +20,18 @@ fn update_mode_enabled() -> bool {
         .unwrap_or(false)
 }
 
-/// Whether the external swift-syntax parser is available. When it is not (e.g.
-/// no Swift toolchain / the `swift-syntax-parse` binary is not on `PATH` and
-/// `CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE` is unset), the corpus test is
-/// skipped rather than failed — it cannot run without the Swift-backed parser.
+/// Whether the external swift-syntax parser is available. When the parser
+/// binary genuinely cannot be found/launched (e.g. no Swift toolchain, and
+/// neither `CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE` nor a `swift-syntax-parse`
+/// on `PATH`), the corpus test is skipped rather than failed — it cannot run
+/// without the Swift-backed parser.
+///
+/// Crucially this checks only that the executable *launches*: a parser that is
+/// present but crashes, emits invalid JSON, or otherwise regresses is
+/// considered available, so the suite runs and fails (rather than silently
+/// skipping the very failures CI needs to catch).
 fn parser_available() -> bool {
-    languages::swift_parse::parse(b"").is_ok()
+    languages::swift_parse::binary_available()
 }
 
 /// Parse a corpus `.output` file. The file holds a single test case made of