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Add an optional hash parameter to builtins.fetchGit
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This is particularly useful to ensure the purity e.g. of a repo which is
pinned to a git tag (tags can be force-pushed which makes builds silently irreproducible)
or when trying to evaluate a nix expression without internet connectivity (if the
cache-entry is expired, the eval will break as the git repo can't be re-fetched).

This change adds an optional `hash` parameter (with an SRI-hash[1]) to the builtin
`fetchGit` and checks if there's a content-addressable path which matches the hash
(the hash can be determined using `nix to-sri --type sha256 $(nix-prefetch-git ...)`).

If no such path exists, it will be attempted to fetch the Git repository in
order to compare the hashes. Please note that caching is still used here, so
if the repo is already fetched and the only hash in the expression changes,
the evaluation will fail pretty fast.

[1] https://www.w3.org/TR/SRI/
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@Ma27
Ma27 committed Nov 14, 2019
1 parent d1db7fa commit 1b1b6ab
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Showing 3 changed files with 94 additions and 14 deletions.
36 changes: 36 additions & 0 deletions doc/manual/expressions/builtins.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -405,6 +405,20 @@ stdenv.mkDerivation { … }
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>hash</term>
<listitem>
<para>
The <link xlink:href="https://www.w3.org/TR/SRI/">SRI</link> hash
of the output path. Please note that this can only
be used in conjunction with <varname>rev</varname> or <varname>ref</varname>.
</para>
<para>
The hash can be generated by running
<command>nix to-sri --type sha256 $(nix-prefetch-url git://github.com/owner/repo --rev abcdef)</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ref</term>
<listitem>
Expand Down Expand Up @@ -504,6 +518,28 @@ stdenv.mkDerivation { … }
<programlisting>builtins.fetchGit {
url = "ssh://git@github.com/nixos/nix.git";
ref = "master";
}</programlisting>
</example>

<example>
<title>Fetching a given revision from a repo and validate its hash</title>
<para>
<function>builtins.fetchGit</function> can create a content-addressable
path to ensure that nothing changes.
</para>
<programlisting>builtins.fetchGit {
url = "git://github.com/NixOS/nix";
rev = "d1db7fa9528154284de55d37e5186ac06fcbffc9";
hash = "sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=";
}</programlisting>
</example>

<example>
<title>Fetching a given tag from a repo and validate its hash</title>
<programlisting>builtins.fetchGit {
url = "git://github.com/NixOS/nix";
ref = "refs/tags/2.2.2";
hash = "sha256-wGVMnUCo8Ms0EdXNlTEAvTBnD/CwOwoAlNCGEijOTJc=";
}</programlisting>
</example>
</listitem>
Expand Down
66 changes: 52 additions & 14 deletions src/libexpr/primops/fetchGit.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -27,11 +27,28 @@ std::regex revRegex("^[0-9a-fA-F]{40}$");

GitInfo exportGit(ref<Store> store, const std::string & uri,
std::optional<std::string> ref, std::string rev,
const std::string & name)
const std::string & name, std::optional<Hash> hash)
{
if (evalSettings.pureEval && rev == "")
throw Error("in pure evaluation mode, 'fetchGit' requires a Git revision");

Path expectedStorePath;
GitInfo gitInfo;
if (hash) {
// Disallow `hash` usage if no rev/ref is given.
if (rev == "" && !ref) {
throw Error("Cannot create a fixed-output derivation for a git repo without a git revision or ref");
}

expectedStorePath = store->makeFixedOutputPath(true, *hash, name);
if (store->isValidPath(expectedStorePath)) {
gitInfo.storePath = expectedStorePath;
gitInfo.rev = rev;
gitInfo.shortRev = std::string(gitInfo.rev, 0, 7);
return gitInfo;
}
}

if (!ref && rev == "" && hasPrefix(uri, "/") && pathExists(uri + "/.git")) {

bool clean = true;
Expand Down Expand Up @@ -138,7 +155,6 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
}

// FIXME: check whether rev is an ancestor of ref.
GitInfo gitInfo;
gitInfo.rev = rev != "" ? rev : chomp(readFile(localRefFile));
gitInfo.shortRev = std::string(gitInfo.rev, 0, 7);

Expand All @@ -148,20 +164,25 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
Path storeLink = cacheDir + "/" + storeLinkName + ".link";
PathLocks storeLinkLock({storeLink}, fmt("waiting for lock on '%1%'...", storeLink)); // FIXME: broken

try {
auto json = nlohmann::json::parse(readFile(storeLink));
// Only return cached git repo if no hash is set. If a hash exists,
// the proper store path would've been returned earlier, otherwise the hash
// needs to be validated.
if (!hash) {
try {
auto json = nlohmann::json::parse(readFile(storeLink));

assert(json["name"] == name && json["rev"] == gitInfo.rev);
assert(json["name"] == name && json["rev"] == gitInfo.rev);

gitInfo.storePath = json["storePath"];
gitInfo.storePath = json["storePath"];

if (store->isValidPath(gitInfo.storePath)) {
gitInfo.revCount = json["revCount"];
return gitInfo;
}
if (store->isValidPath(gitInfo.storePath)) {
gitInfo.revCount = json["revCount"];
return gitInfo;
}

} catch (SysError & e) {
if (e.errNo != ENOENT) throw;
} catch (SysError & e) {
if (e.errNo != ENOENT) throw;
}
}

// FIXME: should pipe this, or find some better way to extract a
Expand All @@ -173,7 +194,21 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,

runProgram("tar", true, { "x", "-C", tmpDir }, tar);

gitInfo.storePath = store->addToStore(name, tmpDir);
auto storePath = store->addToStore(name, tmpDir);
if (expectedStorePath != "" && storePath != expectedStorePath) {
unsigned int status = resUntrustedPath;
Hash got = hashPath(hash->type, store->toRealPath(storePath)).first;
throw Error(
status,
"hash mismatch in git repo (%s, rev %s):\n wanted: %s\n got: %s",
uri,
gitInfo.rev,
hash->to_string(SRI, true),
got.to_string(SRI, true)
);
}

gitInfo.storePath = storePath;

gitInfo.revCount = std::stoull(runProgram("git", true, { "-C", cacheDir, "rev-list", "--count", gitInfo.rev }));

Expand All @@ -194,6 +229,7 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
std::string url;
std::optional<std::string> ref;
std::string rev;
std::optional<Hash> hash;
std::string name = "source";
PathSet context;

Expand All @@ -213,6 +249,8 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
rev = state.forceStringNoCtx(*attr.value, *attr.pos);
else if (n == "name")
name = state.forceStringNoCtx(*attr.value, *attr.pos);
else if (n == "hash")
hash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htUnknown);
else
throw EvalError("unsupported argument '%s' to 'fetchGit', at %s", attr.name, *attr.pos);
}
Expand All @@ -227,7 +265,7 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
// whitelist. Ah well.
state.checkURI(url);

auto gitInfo = exportGit(state.store, url, ref, rev, name);
auto gitInfo = exportGit(state.store, url, ref, rev, name, hash);

state.mkAttrs(v, 8);
mkString(*state.allocAttr(v, state.sOutPath), gitInfo.storePath, PathSet({gitInfo.storePath}));
Expand Down
6 changes: 6 additions & 0 deletions tests/fetchGit.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,12 @@ path3=$(nix eval --raw "(builtins.fetchGit { url = $repo; ref = \"master\"; }).o
path3=$(nix eval --raw "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; }).outPath")
[[ $path = $path3 ]]

path4=$(nix eval --raw "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; hash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG8=\"; }).outPath")
[[ $path = $path4 ]]

nix eval --raw "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; hash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath" || status=$?
[[ "$status" = "102" ]]

# Committing should not affect the store path.
git -C $repo commit -m 'Bla3' -a

Expand Down

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