feat(users/Profpatsch): add atomically-write r/3039

A little shell script to atomically write stdout to a file.

Change-Id: Icca58909c9ad3f92d69af2f5e20c08d69878a77c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3264
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

1 files changed, 28 insertions, 0 deletions

diff --git a/users/Profpatsch/atomically-write.nix b/users/Profpatsch/atomically-write.nix
new file mode 100644
index 0000000000..d5039d3e46
--- /dev/null
+++ b/users/Profpatsch/atomically-write.nix
@@ -0,0 +1,28 @@
+{ depot, pkgs, ... }:
+# Atomically write a file (just `>` redirection in bash
+# empties a file even if the command crashes).
+#
+# Maybe there is an existing tool for that?
+# But it’s easy enough to implement.
+#
+# Example:
+#   atomically-write
+#     ./to
+#     echo "foo"
+#
+# will atomically write the string "foo" into ./to
+let
+  atomically-write = pkgs.writers.writeDash "atomically-write" ''
+    set -e
+    to=$1
+    shift
+    # assumes that the tempfile is on the same file system, (or in memory)
+    # for the `mv` at the end to be more-or-less atomic.
+    tmp=$(${pkgs.coreutils}/bin/mktemp -d)
+    trap 'rm -r "$tmp"' EXIT
+    "$@" \
+      > "$tmp/out"
+    mv "$tmp/out" "$to"
+  '';
+
+in atomically-write