Set SOURCE_DATE_EPOCH to latest source file

This provides a timestamp that's more useful than 1970-01-01 yet still
deterministic.

pkgs/build-support/setup-hooks/set-source-date-epoch-to-latest.sh

@@ -0,0 +1,37 @@
+updateSourceDateEpoch() {
+    local path="$1"
+
+    # FIXME: Support Darwin.
+    if ! [[ $system =~ linux ]]; then
+        exit 1
+        return
+    fi
+
+    # Get the last modification time of all regular files, sort them,
+    # and get the most recent. Maybe we should use
+    # https://github.com/0-wiz-0/findnewest here.
+    local -a res=($(find "$path" -type f -print0 | xargs -0 -r stat -c '%Y %n' | sort -n | tail -n1))
+    local time="${res[0]}"
+    local newestFile="${res[1]}"
+
+    # Update $SOURCE_DATE_EPOCH if the most recent file we found is newer.
+    if [ "$time" -gt "$SOURCE_DATE_EPOCH" ]; then
+        echo "setting SOURCE_DATE_EPOCH to timestamp $time of file $newestFile"
+        export SOURCE_DATE_EPOCH="$time"
+
+        # Warn if the new timestamp is too close to the present. This
+        # may indicate that we were being applied to a file generated
+        # during the build, or that an unpacker didn't restore
+        # timestamps properly.
+        local now="$(date +%s)"
+        if [ "$time" -gt $((now - 60)) ]; then
+            echo "warning: file $newestFile may be generated; SOURCE_DATE_EPOCH may be non-deterministic"
+        fi
+    fi
+}
+
+postUnpackHooks+=(_updateSourceDateEpochFromSourceRoot)
+
+_updateSourceDateEpochFromSourceRoot() {
+    updateSourceDateEpoch "$sourceRoot"
+}

pkgs/stdenv/generic/default.nix

@@ -84,6 +84,7 @@ let
       ../../build-support/setup-hooks/patch-shebangs.sh
       ../../build-support/setup-hooks/move-sbin.sh
       ../../build-support/setup-hooks/move-lib64.sh
+      ../../build-support/setup-hooks/set-source-date-epoch-to-latest.sh
       cc
     ];