pty: cmdbuilder: check for executable access when resolving program

`wezterm start -- /etc/profile` would crash on macOS because
`/etc/profile` isn't executable.

This commit checks for executable access as a prereq for both the
path search and the absolute path cases and generates a non-crashing
error:

```
$ wezterm start -- /etc/profile
17:24:03.574  ERROR  wezterm_gui > Unable to spawn /etc/profile because it doesn't exist on the filesystem or is not executable (EACCES: Permission denied); terminating
```

Cargo.lock

@@ -3305,6 +3305,7 @@ dependencies = [
  "lazy_static",
  "libc",
  "log",
+ "nix 0.24.2",
  "serde",
  "serde_derive",
  "serial",

docs/changelog.md

@@ -19,6 +19,10 @@ As features stabilize some brief notes about them will accumulate here.
 
 * [ActivatePaneDirection](config/lua/keyassignment/ActivatePaneDirection.md) now uses recency to resolve ambiguous moves [#2374](https://github.com/wez/wezterm/issues/2374)
 
+#### Fixed
+
+* macOS: crash on startup if `$SHELL` points to something that isn't executable. [#2378](https://github.com/wez/wezterm/issues/2378)
+
 ### 20220807-113146-c2fee766
 
 #### New

pty/Cargo.toml

@@ -14,6 +14,7 @@ downcast-rs = "1.0"
 filedescriptor = { version="0.8", path = "../filedescriptor" }
 log = "0.4"
 libc = "0.2"
+nix = "0.24"
 shell-words = "1.1"
 serde_derive = {version="1.0", optional=true}
 serde = {version="1.0", optional=true}

pty/src/cmdbuilder.rs

@@ -364,7 +364,9 @@ impl CommandBuilder {
     }
 
     fn search_path(&self, exe: &OsStr, cwd: &OsStr) -> anyhow::Result<OsString> {
+        use nix::unistd::{access, AccessFlags};
         use std::path::Path;
+
         let exe_path: &Path = exe.as_ref();
         if exe_path.is_relative() {
             let cwd: &Path = cwd.as_ref();
@@ -376,7 +378,7 @@ impl CommandBuilder {
             if let Some(path) = self.resolve_path() {
                 for path in std::env::split_paths(&path) {
                     let candidate = path.join(&exe);
-                    if candidate.exists() {
+                    if access(&candidate, AccessFlags::X_OK).is_ok() {
                         return Ok(candidate.into_os_string());
                     }
                 }
@@ -387,9 +389,10 @@ impl CommandBuilder {
                 exe_path.display()
             );
         } else {
-            if !exe_path.exists() {
+            if let Err(err) = access(exe_path, AccessFlags::X_OK) {
                 anyhow::bail!(
-                    "Unable to spawn {} because it doesn't exist on the filesystem",
+                    "Unable to spawn {} because it doesn't exist on the filesystem \
+                    or is not executable ({err:#})",
                     exe_path.display()
                 );
             }