fix: write the dump to a temporary file first

src/scripts/postgres.rs

@@ -1,6 +1,6 @@
 use anyhow::{Context, Result, anyhow, bail};
 use std::{
-    fs::File,
+    fs::{self, File},
     io::{self, Read, Write},
     path::{Path, PathBuf},
     process::Stdio,
@@ -157,6 +157,13 @@ fn restore_cluster(db: &Database, script: &str, file: &Path) -> Result<()> {
     Ok(())
 }
 
+fn suffixed(path: &Path, suffix: &str) -> PathBuf {
+    let mut out = path.as_os_str().to_owned();
+    out.push(suffix);
+
+    PathBuf::from(out)
+}
+
 fn read_header(path: &Path) -> Result<Vec<u8>> {
     let mut file = File::open(path).with_context(|| format!("opening {}", path.display()))?;
     let mut header = vec![0; HEADER_LEN];
@@ -343,15 +350,21 @@ pub fn dump(file: &PathBuf, format: Format, gzip: bool) -> Result<()> {
 
     eprintln!("dumping to local file {}", file.to_string_lossy());
 
-    let stdout = Stdio::from(File::create(file)?);
-    if gzip {
+    // written beside the target and renamed once the dump succeeds, so a failure
+    // cannot destroy the dump that is already there
+    let partial = suffixed(file, ".partial");
+    let stdout = Stdio::from(
+        File::create(&partial).with_context(|| format!("creating {}", partial.display()))?,
+    );
+
+    let dumped = if gzip {
         // the whole pipeline has to arrive as one shell argument
         CommandBuilder::docker()
             .args("exec")
             .arg(&db.container)
             .args("sh -c")
             .arg(format!("{} | gzip", dump_command(&db, format)))
-            .exec_redirect_stdout(stdout)?;
+            .exec_redirect_stdout(stdout)
     } else {
         let dumping = match format.flag() {
             Some(flag) => in_container(&db)
@@ -362,9 +375,17 @@ pub fn dump(file: &PathBuf, format: Format, gzip: bool) -> Result<()> {
             None => in_container(&db).args("pg_dumpall -U").arg(&db.user),
         };
 
-        dumping.exec_redirect_stdout(stdout)?;
+        dumping.exec_redirect_stdout(stdout)
+    };
+
+    if let Err(e) = dumped {
+        let _ = fs::remove_file(&partial);
+        return Err(e);
     }
 
+    fs::rename(&partial, file)
+        .with_context(|| format!("renaming {} to {}", partial.display(), file.display()))?;
+
     Ok(())
 }