write_file: Do not overwrite with same content

The `write_file` function will no longer overwrite existing files if the
content has not changed.  This will ensure file metadata timestamps are
accurate, and reduce unnecessary filesystem activity.

Cargo.lock

@@ -300,6 +300,12 @@ version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
 
+[[package]]
+name = "hex"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
+
 [[package]]
 name = "humansize"
 version = "2.1.3"
@@ -923,6 +929,7 @@ dependencies = [
  "argparse",
  "blake2",
  "file-mode",
+ "hex",
  "pwd-grp",
  "serde",
  "serde_yaml",

Cargo.toml

@@ -9,6 +9,7 @@ edition = "2021"
 argparse = "0.2.2"
 blake2 = "0.10.6"
 file-mode = { version = "0.1.2", features = ["serde"] }
+hex = "0.4.3"
 pwd-grp = "0.1.1"
 serde = { version = "1.0.195", features = ["derive"] }
 serde_yaml = "0.9.30"

src/main.rs

@@ -12,7 +12,7 @@ use serde::de::DeserializeOwned;
 use serde_yaml::Value;
 use shlex::Shlex;
 use tera::{Context, Tera};
-use tracing::{debug, error, info, warn};
+use tracing::{debug, error, info, trace, warn};
 
 use model::Instructions;
 
@@ -161,13 +161,18 @@ fn process_instructions(
         };
         let mut dest = PathBuf::from(destdir.as_ref());
         dest.push(i.dest.strip_prefix("/").unwrap_or(i.dest.as_path()));
-        let orig_cksm = checksum(&dest).ok();
-        if let Err(e) = write_file(&dest, out.as_bytes()) {
-            error!("Failed to write output file {}: {}", dest.display(), e);
-            continue;
-        }
-        let new_cksm = checksum(&dest).ok();
-        if orig_cksm != new_cksm {
+        let changed = match write_file(&dest, out.as_bytes()) {
+            Ok(c) => c,
+            Err(e) => {
+                error!(
+                    "Failed to write output file {}: {}",
+                    dest.display(),
+                    e
+                );
+                continue;
+            }
+        };
+        if changed {
             info!("File {} was changed", dest.display());
             if let Some(hooks) = i.hooks {
                 if let Some(changed) = hooks.changed {
@@ -219,19 +224,37 @@ fn process_instructions(
 fn write_file(
     dest: impl AsRef<Path>,
     data: &[u8],
-) -> Result<(), std::io::Error> {
+) -> Result<bool, std::io::Error> {
     if let Some(p) = dest.as_ref().parent() {
         if !p.exists() {
             info!("Creating directory {}", p.display());
             std::fs::create_dir_all(p)?;
         }
     }
+    if let Ok(orig_cksm) = checksum(&dest) {
+        trace!(
+            "Original checksum: {}: {}",
+            dest.as_ref().display(),
+            hex::encode(&orig_cksm)
+        );
+        let mut blake = Blake2b512::new();
+        blake.update(data);
+        let new_cksm = blake.finalize().to_vec();
+        trace!(
+            "New checksum: {}: {}",
+            dest.as_ref().display(),
+            hex::encode(&new_cksm)
+        );
+        if orig_cksm == new_cksm {
+            return Ok(false);
+        }
+    }
     debug!("Writing output: {}", dest.as_ref().display());
     let mut f = std::fs::File::create(&dest)?;
     f.write_all(data)?;
     let size = f.stream_position()?;
     debug!("Wrote output: {} ({} bytes)", dest.as_ref().display(), size);
-    Ok(())
+    Ok(true)
 }
 
 fn chown(