install: stop nested --bun from self-referencing its node shim

src/install/lib.rs

@@ -577,27 +577,56 @@ impl RunCommand {
 
             let argv0: &ZStr = bun_core::argv().get(0).unwrap_or(bun_core::zstr!("bun"));
 
-            // if we are already an absolute path, use that
-            // if the user started the application via a shebang, it's likely that the path is absolute already
-            let argv0_z: &ZStr = if argv0.as_bytes().first() == Some(&b'/') {
-                *optional_bun_path = argv0.as_bytes();
-                argv0
-            } else if optional_bun_path.is_empty() {
-                // otherwise, ask the OS for the absolute path
-                let self_path = bun_core::self_exe_path()?;
-                if !self_path.as_bytes().is_empty() {
-                    *optional_bun_path = self_path.as_bytes();
-                    self_path
-                } else {
-                    argv0
-                }
-            } else {
-                // When argv[0] is
-                // not absolute and the caller pre-supplied a path, that path is the
-                // symlink target (NOT argv[0]).
+            // PREFER `self_exe_path()` OVER `argv[0]`: on a nested `--bun`, the
+            // OUTER bun prepends `BUN_NODE_DIR` to `PATH` and the INNER bun is
+            // execve'd with `argv[0] = <BUN_NODE_DIR>/bun` — exactly the shim
+            // we're about to (re)write. Using that as the symlink target
+            // produces `<BUN_NODE_DIR>/bun -> <BUN_NODE_DIR>/bun` (self-loop),
+            // and the next `/usr/bin/env node` bails with ELOOP "Too many
+            // levels of symbolic links" (#30711). `self_exe_path()` readlinks
+            // `/proc/self/exe` (Linux) / canonicalizes `_NSGetExecutablePath`
+            // (macOS), so it always resolves to the REAL bun regardless of
+            // how the process was invoked. It's memoized via `Once`, so the
+            // cost is paid once per process.
+            let argv0_z: &ZStr = if !optional_bun_path.is_empty() {
+                // When the caller pre-supplied a path, that path is the symlink
+                // target.
                 // SAFETY: callers pass a slice borrowed from a `ZStr` (argv[0] /
                 // self_exe_path / static literal), so `ptr[len] == 0` holds.
                 unsafe { ZStr::from_raw(optional_bun_path.as_ptr(), optional_bun_path.len()) }
+            } else {
+                // Ask the OS for the real absolute path first. Fall back to an
+                // absolute `argv[0]` only if that fails — never trust a bare
+                // `argv[0]` as the target here, because on nested `--bun` the
+                // inner process's `argv[0]` IS `<BUN_NODE_DIR>/bun`.
+                match bun_core::self_exe_path() {
+                    Ok(self_path) if !self_path.as_bytes().is_empty() => {
+                        *optional_bun_path = self_path.as_bytes();
+                        self_path
+                    }
+                    result => {
+                        let argv0_bytes = argv0.as_bytes();
+                        if argv0_bytes.starts_with(Self::BUN_NODE_DIR.as_bytes()) {
+                            // `self_exe_path()` failed and `argv[0]` is the shim
+                            // under `BUN_NODE_DIR` (nested `--bun`). Using it as
+                            // the target would recreate the #30711 self-loop; the
+                            // OUTER bun already planted working shims and PATH, so
+                            // leave them untouched.
+                            return Ok(());
+                        }
+                        if argv0_bytes.first() == Some(&b'/') {
+                            *optional_bun_path = argv0_bytes;
+                            argv0
+                        } else {
+                            // No usable target — propagate the OS error when we
+                            // have one, otherwise leave PATH unmodified.
+                            return match result {
+                                Err(e) => Err(e),
+                                Ok(_) => Ok(()),
+                            };
+                        }
+                    }
+                }
             };
 
             #[cfg(bun_debug)]

test/cli/install/bun-run.test.ts

@@ -1068,4 +1068,55 @@ describe.concurrent("bun run", () => {
       expect(exitCode).toBe(0);
     },
   );
+
+  // https://github.com/oven-sh/bun/issues/30711 — nested `--bun` used to rewrite
+  // the BUN_NODE_DIR/{bun,node} shim to point at ITSELF. After the OUTER `--bun`
+  // prepends BUN_NODE_DIR to PATH, the INNER bun's argv[0] (PATH-resolved) is
+  // `<BUN_NODE_DIR>/bun` — exactly the shim it was about to rewrite. The symlink
+  // becomes self-referencing and `/usr/bin/env node` fails with ELOOP
+  // ("Too many levels of symbolic links").
+  it.skipIf(isWindows)("nested --bun does not create a self-referencing node/bun shim", async () => {
+    // Reproduce the reporter's exact invocation: `bun run --bun bun run --bun <binScript>`
+    // where:
+    // - the literal "bun" must be resolved via PATH so the inner process's
+    //   argv[0] ends up being the shim path (this is what feeds the self-loop
+    //   into the symlink target)
+    // - the final script must be a `node_modules/.bin/<name>` shim script
+    //   that shebang's `/usr/bin/env node` — that's where ELOOP actually
+    //   surfaces when the shim is broken
+    using dir = tempDir("bun-run-nested-bun-shim", {
+      "package.json": JSON.stringify({ name: "nested-bun-shim" }),
+      "node_modules/fake-pkg/bin.js": "#!/usr/bin/env node\nconsole.log('nested --bun ok');\n",
+    });
+
+    const dirStr = String(dir);
+    chmodSync(join(dirStr, "node_modules/fake-pkg/bin.js"), 0o755);
+
+    // node_modules/.bin/fake-pkg → ../fake-pkg/bin.js
+    const binDir = join(dirStr, "node_modules/.bin");
+    await $`mkdir -p ${binDir} && ln -sf ../fake-pkg/bin.js ${binDir}/fake-pkg`.quiet();
+
+    // Plant a `bun` symlink on PATH so the outer `--bun bun` resolves via
+    // PATH-lookup (not as an absolute argv[0]).
+    const pathBinDir = join(dirStr, "path-bin");
+    await $`mkdir -p ${pathBinDir} && ln -sf ${bunExe()} ${pathBinDir}/bun`.quiet();
+
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "run", "--bun", "bun", "run", "--bun", "fake-pkg"],
+      cwd: dirStr,
+      env: { ...bunEnv, PATH: `${pathBinDir}:${bunEnv.PATH ?? process.env.PATH}` },
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    // Buggy versions emit "env: 'node': Too many levels of symbolic links"
+    // and exit 126; the fixed version runs the script cleanly.
+    expect({ stderr, stdout, exitCode }).toEqual({
+      stderr: expect.not.stringContaining("Too many levels of symbolic links"),
+      stdout: expect.stringContaining("nested --bun ok"),
+      exitCode: 0,
+    });
+  });
 });