Replaces 'flock' with mkdir locking (#886)

This PR replaces the Linux-specific `flock` command with a portable
directory-based locking in the `test.sh` script. The change ensures
compatibility across different Unix-like systems, including macOS, while
maintaining the same functionality.

Testing
The locking mechanism has been verified with the following test cases:
1. Running a single instance of the test script
2. Attempting to run multiple instances concurrently
3. Testing cleanup on normal script completion
4. Testing cleanup on interruption (SIGINT, SIGTERM)

Closes [#878](payjoin/rust-payjoin#878)

Author: node-smithxby72w

contrib/test.sh

@@ -2,48 +2,52 @@
 set -e
 
 LOCKFILE="Cargo.lock"
-LOCKFILE_BAK="Cargo.lock.bak"
-LOCKFILE_LOCK=".Cargo.lock.flock"
+LOCKDIR=".bak"
+LOCKFILE_BAK="${LOCKDIR}/${LOCKFILE}"
 
-# Acquire file lock to prevent concurrent modification
-# We can't use cargo test --lockfile-path since that doesn't exist in our MSRV
-(
-    flock 9
+cleanup() {
+    if [ -f "$LOCKFILE_BAK" ]; then
+        mv "$LOCKFILE_BAK" "$LOCKFILE"
+    fi
+    rmdir "$LOCKDIR"
+}
+
+if ! mkdir "$LOCKDIR" 2>/dev/null; then
+    echo "Another instance is running. If you're sure it's not, remove $LOCKDIR and try again." >&2
+    exit 1
+fi
+
+trap cleanup EXIT
+
+if [ -f "$LOCKFILE" ]; then
+    mv "$LOCKFILE" "$LOCKFILE_BAK"
+fi
 
-    # Backup original lockfile
-    if [ -f "$LOCKFILE" ]; then
-        mv "$LOCKFILE" "$LOCKFILE_BAK"
+DEPS="recent minimal"
+CRATES="payjoin payjoin-cli payjoin-directory payjoin-ffi"
+
+for dep in $DEPS; do
+    cargo --version
+    rustc --version
+
+    # Some tests require certain toolchain types.
+    export NIGHTLY=false
+    export STABLE=true
+    if cargo --version | grep nightly; then
+        STABLE=false
+        NIGHTLY=true
     fi
+    if cargo --version | grep beta; then
+        STABLE=false
+    fi
+
+    cp "Cargo-$dep.lock" "$LOCKFILE"
 
-    # Restore the original lockfile on exit
-    trap '[ -f "$LOCKFILE_BAK" ] && mv "$LOCKFILE_BAK" "$LOCKFILE"' EXIT
-
-    DEPS="recent minimal"
-    CRATES="payjoin payjoin-cli payjoin-directory payjoin-ffi"
-
-    for dep in $DEPS; do
-        cargo --version
-        rustc --version
-
-        # Some tests require certain toolchain types.
-        export NIGHTLY=false
-        export STABLE=true
-        if cargo --version | grep nightly; then
-            STABLE=false
-            NIGHTLY=true
-        fi
-        if cargo --version | grep beta; then
-            STABLE=false
-        fi
-
-        cp "Cargo-$dep.lock" Cargo.lock
-
-        for crate in $CRATES; do
-            (
-                cd "$crate"
-                ./contrib/test.sh
-            )
-        done
+    for crate in $CRATES; do
+        (
+            cd "$crate"
+            ./contrib/test.sh
+        )
     done
+done
 
-) 9>"$LOCKFILE_LOCK"