From 8a29232801708e2679ee1f444136d76e6e340837 Mon Sep 17 00:00:00 2001
From: Andronik Ordian <write@reusable.software>
Date: Wed, 25 Mar 2020 18:46:49 +0100
Subject: [PATCH] Ban duplicates of parity-uil-mem from being linked into the
 same program (#363)

* Prevent multiple versions of parity-uil-mem from being linked into the same program

* fmt

* parity-util-mem: add a warning about defining global allocs

* parity-util-mem: document empty build script

* parity-util-mem: extend the warning based on David comments
---
 parity-util-mem/CHANGELOG.md |  2 ++
 parity-util-mem/Cargo.toml   |  7 +++++++
 parity-util-mem/README.md    | 14 ++++++++++++++
 parity-util-mem/build.rs     |  1 +
 4 files changed, 24 insertions(+)
 create mode 100644 parity-util-mem/build.rs

diff --git a/parity-util-mem/CHANGELOG.md b/parity-util-mem/CHANGELOG.md
index fdb3f5f7b..ec8ef91f9 100644
--- a/parity-util-mem/CHANGELOG.md
+++ b/parity-util-mem/CHANGELOG.md
@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog].
 [Keep a Changelog]: http://keepachangelog.com/en/1.0.0/
 
 ## [Unreleased]
+### Breaking
+- Prevent multiple versions from being linked into the same program. [#363](https://github.com/paritytech/parity-common/pull/363)
 
 ## [0.6.0] - 2020-03-13
 - Updated dependencies. [#361](https://github.com/paritytech/parity-common/pull/361)
diff --git a/parity-util-mem/Cargo.toml b/parity-util-mem/Cargo.toml
index af3916902..700061c32 100644
--- a/parity-util-mem/Cargo.toml
+++ b/parity-util-mem/Cargo.toml
@@ -7,6 +7,13 @@ description = "Collection of memory related utilities"
 license = "MIT OR Apache-2.0"
 edition = "2018"
 
+# Prevent multiple versions from being linked into the same program.
+links = "parity-util-mem-ban-duplicates"
+# `links` requires a build script to be present:
+# https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key
+# so we use an empty build script
+build = "build.rs"
+
 [dependencies]
 cfg-if = "0.1.10"
 dlmalloc = { version = "0.1.3", features = ["global"], optional = true }
diff --git a/parity-util-mem/README.md b/parity-util-mem/README.md
index 14d6e6c19..a6f15b90b 100644
--- a/parity-util-mem/README.md
+++ b/parity-util-mem/README.md
@@ -2,6 +2,20 @@
 
 Collection of memory related utilities.
 
+## WARNING
+
+When `parity-util-mem` is used as a dependency with any of the global allocator features enabled,
+it must be the sole place where a global allocator is defined.
+The only exception to this rule is when used in a `no_std` context or when the `estimate-heapsize` feature is used.
+
+Because of that, it must be present in the dependency tree with a single version.
+Starting from version 0.7, having duplicate versions of `parity-util-mem` will lead
+to a compile-time error. It still will be possible to have 0.6 and 0.7 versions in the same binary though.
+
+Unless heeded you risk UB; see discussion in [issue 364].
+
+[issue 364]: https://github.com/paritytech/parity-common/issues/364
+
 ## Features
 
 - estimate-heapsize : Do not use allocator, but `size_of` or `size_of_val`.
diff --git a/parity-util-mem/build.rs b/parity-util-mem/build.rs
new file mode 100644
index 000000000..f328e4d9d
--- /dev/null
+++ b/parity-util-mem/build.rs
@@ -0,0 +1 @@
+fn main() {}