retimer.sh

#!/bin/bash
# `retimer` saves modification times (mtimes) of source files, and restores them only if the
# contents match a hash.
#
# `retimer`'s intended workflow is as follows:
#
# 1. restore `.retimer-state` and `target` from CI cache, if available
# 2. `retimer restore`
# 3. `cargo build`
# 4. `retimer save`
# 5. save `.retimer-state` and `target` in CI cache for the next run
#
# This allows `cargo`'s mtime-based caching to work properly with CI/git.
#
# Based on scottlamb's https://github.com/rust-lang/cargo/issues/6529#issuecomment-1558438048

set -euo pipefail

RETIMER_STATE_FILE=".retimer-state"

# Function to compute SHA256 hash of a file
compute_hash() {
    local file="$1"
    if command -v sha256sum >/dev/null 2>&1; then
        sha256sum "$file" | cut -d' ' -f1
    elif command -v shasum >/dev/null 2>&1; then
        shasum -a 256 "$file" | cut -d' ' -f1
    else
        echo "Error: Neither sha256sum nor shasum found" >&2
        exit 1
    fi
}

# Function to get modification time in seconds since epoch
get_mtime() {
    local file="$1"
    if [[ "$OSTYPE" == "darwin"* ]]; then
        stat -f %m "$file"
    else
        stat -c %.Y "$file"
    fi
}

# Function to set modification time
set_mtime() {
    local file="$1"
    local mtime="$2"

    # Try different approaches based on OS
    if [[ "$OSTYPE" == "darwin"* ]]; then
        # macOS: use date -r with timestamp
        touch -t "$(date -r "$mtime" +%Y%m%d%H%M.%S)" "$file" 2>/dev/null || \
        touch -d "@$mtime" "$file" 2>/dev/null || {
            echo "Warning: Could not set mtime for $file" >&2
            return 1
        }
    else
        touch -d "@$mtime" "$file" 2>/dev/null || {
            echo "Warning: Could not set mtime for $file" >&2
            return 1
        }
    fi
}

# Function to save current state
save_state() {
    echo "Saving retimer state..."
    local temp_file="${RETIMER_STATE_FILE}.tmp"

    # Create temporary file
    > "$temp_file"

    # Find all src files and save their hash and mtime
    find . -name "*.rs" -type f -not -path "./target/*" | while IFS= read -r file; do
        if [[ -r "$file" ]]; then
            local hash
            local mtime
            hash=$(compute_hash "$file")
            mtime=$(get_mtime "$file")
            printf "%s\t%s\t%s\n" "$file" "$hash" "$mtime" >> "$temp_file"
        fi
    done

    # Atomically replace the state file
    mv "$temp_file" "$RETIMER_STATE_FILE"

    local count
    count=$(wc -l < "$RETIMER_STATE_FILE")
    echo "Saved state for $count files"
}

# Function to restore state
restore_state() {
    if [[ ! -f "$RETIMER_STATE_FILE" ]]; then
        echo "No retimer state file found, skipping restore"
        return 0
    fi

    echo "Restoring retimer state..."
    local restored=0
    local skipped=0
    local total=0

    while IFS= read -r line; do
        total=$((total + 1))

        # Parse tab-separated values manually to avoid shell escaping issues
        local file hash saved_mtime
        file=$(echo "$line" | cut -f1)
        hash=$(echo "$line" | cut -f2)
        saved_mtime=$(echo "$line" | cut -f3)

        # Skip empty lines or malformed entries
        if [[ -z "$file" || -z "$hash" || -z "$saved_mtime" ]]; then
            echo "Warning: Malformed line in state file, skipping"
            skipped=$((skipped + 1))
            continue
        fi

        if [[ ! -f "$file" ]]; then
            echo "Warning: File $file no longer exists, skipping"
            skipped=$((skipped + 1))
            continue
        fi

        if [[ ! -r "$file" ]]; then
            echo "Warning: Cannot read $file, skipping"
            skipped=$((skipped + 1))
            continue
        fi

        local current_hash
        current_hash=$(compute_hash "$file")

        if [[ "$current_hash" == "$hash" ]]; then
            if set_mtime "$file" "$saved_mtime"; then
                restored=$((restored + 1))
            else
                echo "Warning: Failed to set mtime for $file"
                skipped=$((skipped + 1))
            fi
        else
            # File content has changed, don't restore mtime
            echo "Skipping $file (content changed: $current_hash vs $hash)"
            skipped=$((skipped + 1))
        fi
    done < "$RETIMER_STATE_FILE"

    echo "Restored $restored files, skipped $skipped files (out of $total total)"
}

# Function to show usage
usage() {
    echo "Usage: $0 {save|restore}"
    echo "  save    - Save current mtimes and hashes of .rs files"
    echo "  restore - Restore mtimes for files whose content hasn't changed"
    exit 1
}

# Main script logic
case "${1:-}" in
    save)
        save_state
        ;;
    restore)
        restore_state
        ;;
    *)
        usage
        ;;
esac