Make libfuse.dylib a weak runtime dependency on macOS

Add weak linking for libfuse on macOS so the binary can load even when
macFUSE is not installed. The mount command now checks for libfuse
availability at runtime using dlopen and provides a helpful error message
if macFUSE is missing.

Author: penberg

cli/build.rs

@@ -7,4 +7,12 @@ fn main() {
         // libgcc_s provides _Unwind_RaiseException and other exception handling symbols
         println!("cargo:rustc-link-lib=dylib=gcc_s");
     }
+
+    // macOS: Weak-link libfuse so the binary can load without macFUSE installed.
+    #[cfg(target_os = "macos")]
+    {
+        println!("cargo:rustc-link-arg=-Wl,-weak-lfuse");
+        println!("cargo:rustc-link-search=/usr/local/lib");
+        println!("cargo:rustc-link-search=/Library/Frameworks/macFUSE.framework/Versions/A");
+    }
 }

cli/src/cmd/mount.rs

@@ -25,6 +25,20 @@ pub struct MountArgs {
 
 /// Mount the agent filesystem using FUSE.
 pub fn mount(args: MountArgs) -> Result<()> {
+    if !supports_fuse() {
+        #[cfg(target_os = "macos")]
+        {
+            anyhow::bail!(
+                "macFUSE is not installed. Please install it from https://osxfuse.github.io/ \n\
+                 or via Homebrew: brew install --cask macfuse"
+            );
+        }
+        #[cfg(not(target_os = "macos"))]
+        {
+            anyhow::bail!("FUSE is not available on this system");
+        }
+    }
+
     let opts = AgentFSOptions::resolve(&args.id_or_path)?;
 
     let fsname = std::fs::canonicalize(&args.id_or_path)
@@ -83,3 +97,28 @@ fn is_mounted(path: &std::path::Path) -> bool {
     // Different device IDs means it's a mountpoint
     path_meta.dev() != parent_meta.dev()
 }
+
+/// Check if macOS system supports FUSE.
+///
+/// The `libfuse` dynamic library is weakly linked so that users who don't have
+/// macFUSE installed can still run the other commands.
+#[cfg(target_os = "macos")]
+fn supports_fuse() -> bool {
+    for lib_name in &[c"libfuse.2.dylib", c"libfuse.dylib"] {
+        let handle = unsafe { libc::dlopen(lib_name.as_ptr(), libc::RTLD_LAZY) };
+        if !handle.is_null() {
+            unsafe { libc::dlclose(handle) };
+            return true;
+        }
+    }
+    false
+}
+
+/// Check if Linux system supports FUSE.
+///
+/// The `fuser` crate does not even need `libfuse` so technically it always support FUSE.
+/// Of course, if FUSE is disabled in the kernel, we'll get an error, but that's life.
+#[cfg(target_os = "linux")]
+fn supports_fuse() -> bool {
+    true
+}