Prevent concurrent lockfile conflicts using flock and add backup (payjoin#851)

 ## Summary
This PR introduces a safe and stable mechanism to manage Cargo.lock
during testing across multiple scripts.

- File Locking with flock: Ensures only one script can manipulate
Cargo.lock at a time using a .Cargo.lock.flock file.
- Backup and Restore: test.sh creates a backup of the existing
Cargo.lock before overwriting it and restores it afterward using an EXIT
trap.

This prevents loss of local Cargo.lock changes when running ./contrib/test.sh, and race conditions when running
concurrently

closes payjoin#773

Author: nothingmuch

contrib/test.sh

@@ -1,30 +1,49 @@
 #!/usr/bin/env bash
 set -e
 
-DEPS="recent minimal"
-CRATES="payjoin payjoin-cli payjoin-directory payjoin-ffi"
-
-for dep in $DEPS; do
-    cargo --version
-    rustc --version
-
-    # Some tests require certain toolchain types.
-    export NIGHTLY=false
-    export STABLE=true
-    if cargo --version | grep nightly; then
-        STABLE=false
-        NIGHTLY=true
-    fi
-    if cargo --version | grep beta; then
-        STABLE=false
+LOCKFILE="Cargo.lock"
+LOCKFILE_BAK="Cargo.lock.bak"
+LOCKFILE_LOCK=".Cargo.lock.flock"
+
+# Acquire file lock to prevent concurrent modification
+# We can't use cargo test --lockfile-path since that doesn't exist in our MSRV
+(
+    flock 9
+
+    # Backup original lockfile
+    if [ -f "$LOCKFILE" ]; then
+        mv "$LOCKFILE" "$LOCKFILE_BAK"
     fi
 
-    cp "Cargo-$dep.lock" Cargo.lock
+    # Restore the original lockfile on exit
+    trap '[ -f "$LOCKFILE_BAK" ] && mv "$LOCKFILE_BAK" "$LOCKFILE"' EXIT
+
+    DEPS="recent minimal"
+    CRATES="payjoin payjoin-cli payjoin-directory payjoin-ffi"
 
-    for crate in $CRATES; do
-        (
-            cd "$crate"
-            ./contrib/test.sh
-        )
+    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
     done
-done
+
+) 9>"$LOCKFILE_LOCK"