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nix-copy-closure: Fix race condition

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There is a long-standing race condition when copying a closure to a
remote machine, particularly affecting build-remote.pl: the client
first asks the remote machine which paths it already has, then copies
over the missing paths. If the garbage collector kicks in on the
remote machine between the first and second step, the already-present
paths may be deleted. The missing paths may then refer to deleted
paths, causing nix-copy-closure to fail. The client now performs both
steps using a single remote Nix call (using ‘nix-store --serve’),
locking all paths in the closure while querying.

I changed the --serve protocol a bit (getting rid of QueryCommand), so
this breaks the SSH substituter from older versions. But it was marked
experimental anyway.

Fixes #141.
This commit is contained in:
Eelco Dolstra 2014-07-10 11:51:22 +02:00
parent 2c3a8f787b
commit 04170d06bf
4 changed files with 137 additions and 61 deletions
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8
src/download-via-ssh/download-via-ssh.cc
View file

@ -58,7 +58,7 @@ static std::pair<FdSink, FdSource> connect(const string & conn)
static void substitute(std::pair<FdSink, FdSource> & pipes, Path storePath, Path destPath)
{
writeInt(cmdSubstitute, pipes.first);
writeInt(cmdDumpStorePath, pipes.first);
writeString(storePath, pipes.first);
pipes.first.flush();
restorePath(destPath, pipes.second);
@ -68,20 +68,20 @@ static void substitute(std::pair<FdSink, FdSource> & pipes, Path storePath, Path
static void query(std::pair<FdSink, FdSource> & pipes)
{
writeInt(cmdQuery, pipes.first);
for (string line; getline(std::cin, line);) {
Strings tokenized = tokenizeString<Strings>(line);
string cmd = tokenized.front();
tokenized.pop_front();
if (cmd == "have") {
writeInt(qCmdHave, pipes.first);
writeInt(cmdQueryValidPaths, pipes.first);
writeInt(0, pipes.first); // don't lock
writeStrings(tokenized, pipes.first);
pipes.first.flush();
PathSet paths = readStrings<PathSet>(pipes.second);
foreach (PathSet::iterator, i, paths)
std::cout << *i << std::endl;
} else if (cmd == "info") {
writeInt(qCmdInfo, pipes.first);
writeInt(cmdQueryPathInfos, pipes.first);
writeStrings(tokenized, pipes.first);
pipes.first.flush();
while (1) {