Remove nanosecond modification times from Cargo fingerprints

compiler/base/Dockerfile

@@ -3,6 +3,7 @@ FROM ubuntu:14.04
 RUN apt-get update && apt-get install -y \
     curl \
     gcc \
+    jq \
     libssl-dev \
  && rm -rf /var/lib/apt/lists/*
 
@@ -11,3 +12,4 @@ ENV PATH=/root/.cargo/bin:$PATH
 RUN curl https://sh.rustup.rs -sSf | sh -s -- -y  && rustup toolchain uninstall stable
 ADD Cargo.toml /root/Cargo.toml
 ADD entrypoint.sh /root/
+ADD fix-modification-time.sh /root/

compiler/base/fix-modification-time.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+set -eu -o pipefail
+
+# The docker filesystem layers (at least AUFS) preserve
+# nanosecond-level granularity while the container is running, but
+# lose that granularity when the image is saved.
+#
+# This causes spurious rebuilds of any crate that uses file
+# modification-based timestamps, as the time appears to have
+# changed. See https://github.com/rust-lang/cargo/issues/2874 for some
+# further details.
+#
+# As a terrible, nasty hack, let's just rewrite the fingerprint files
+# to set the nanosecond value to 0, matching the filesystem. This
+# relies on the unstable internals of the JSON cache files, so this is
+# likely quite brittle.
+
+t=$(mktemp)
+jq 'if .local.variant == "MtimeBased" then .local.fields[0][1] |= 0 else . end' "$1" > "$t"
+mv "$t" "$1"

compiler/beta/Dockerfile

@@ -12,4 +12,6 @@ RUN mv /root/Cargo.toml ./Cargo.toml && \
     cargo build && \
     cargo build --release
 
+RUN find . -name '*.json' -print0 | xargs -0 -n1 /root/fix-modification-time.sh
+
 ENTRYPOINT ["/root/entrypoint.sh"]

compiler/nightly/Dockerfile

@@ -12,4 +12,6 @@ RUN mv /root/Cargo.toml ./Cargo.toml && \
     cargo build && \
     cargo build --release
 
+RUN find . -name '*.json' -print0 | xargs -0 -n1 /root/fix-modification-time.sh
+
 ENTRYPOINT ["/root/entrypoint.sh"]

compiler/stable/Dockerfile

@@ -12,4 +12,6 @@ RUN mv /root/Cargo.toml ./Cargo.toml && \
     cargo build && \
     cargo build --release
 
+RUN find . -name '*.json' -print0 | xargs -0 -n1 /root/fix-modification-time.sh
+
 ENTRYPOINT ["/root/entrypoint.sh"]