Using the immutable bit is problematic, especially in conjunction with
store optimisation. For instance, if the garbage collector deletes a
file, it has to clear its immutable bit, but if the file has
additional hard links, we can't set the bit afterwards because we
don't know the remaining paths.
So now that we support having the entire Nix store as a read-only
mount, we may as well drop the immutable bit. Unfortunately, we have
to keep the code to clear the immutable bit for backwards
compatibility.
It turns out that the immutable bit doesn't work all that well. A
better way is to make the entire Nix store a read-only bind mount,
i.e. by doing
$ mount --bind /nix/store /nix/store
$ mount -o remount,ro,bind /nix/store
(This would typically done in an early boot script, before anything
from /nix/store is used.)
Since Nix needs to be able to write to the Nix store, it now detects
if /nix/store is a read-only bind mount and then makes it writable in
a private mount namespace.
The outputs of a derivation can refer to each other (even though they
cannot have cycles), so they have to be deleted in the right order.
http://hydra.nixos.org/build/3026118
If the options gc-keep-outputs and gc-keep-derivations are both
enabled, you can get a cycle in the liveness graph. There was a hack
to handle this, but it didn't work with multiple-output derivations,
causing the garbage collector to fail with errors like ‘error: cannot
delete path `...' because it is in use by `...'’. The garbage
collector now handles strongly connected components in the liveness
graph as a unit and decides whether to delete all or none of the paths
in an SCC.
Note that this will only work if the client has a very recent Nix
version (post 15e1b2c223), otherwise the
--option flag will just be ignored.
Fixes#50.
This handles the chroot and build hook cases, which are easy.
Supporting the non-chroot-build case will require more work (hash
rewriting!).
Issue #21.
"config.h" must be included first, because otherwise the compiler
might not see the right value of _FILE_OFFSET_BITS. We've had this
before; see 705868a8a9. In this case,
GCC would compute a different address for ‘settings.useSubstitutes’ in
misc.cc because of the off_t in ‘settings’.
Reverts 3854fc9b42.
http://hydra.nixos.org/build/3016700
Output names are now appended to resulting GC symlinks, e.g. by
nix-build. For backwards compatibility, if the output is named "out",
nothing is appended. E.g. doing "nix-build -A foo" on a derivation
that produces outputs "out", "bin" and "dev" will produce symlinks
"./result", "./result-bin" and "./result-dev", respectively.
This is required on systemd, which mounts filesystems as "shared"
subtrees. Changes to shared trees in a private mount namespace are
propagated to the outside world, which is bad.
More precisely, in concatLists, if all lists except one are empty,
then just return the non-empty list. This reduces the number of list
element allocations by 32% when evaluating a NixOS system
configuration.
This can serve as a generic efficient list builder. For instance, the
function ‘catAttrs’ in Nixpkgs can be rewritten from
attr: l: fold (s: l: if hasAttr attr s then [(getAttr attr s)] ++ l else l) [] l
to
attr: l: builtins.concatLists (map (s: if hasAttr attr s then [(getAttr attr s)] else []) l)
Statistics before:
time elapsed: 1.08683
size of a value: 24
environments allocated: 1384376 (35809568 bytes)
list elements: 6946783 (55574264 bytes)
list concatenations: 37434
values allocated: 1760440 (42250560 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18273
number of thunks: 1297673
number of thunks avoided: 1380759
number of attr lookups: 430802
number of primop calls: 628912
number of function calls: 1333544
Statistics after (including new catAttrs):
time elapsed: 0.959854
size of a value: 24
environments allocated: 1010198 (26829296 bytes)
list elements: 1984878 (15879024 bytes)
list concatenations: 30488
values allocated: 1589760 (38154240 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18274
number of thunks: 1040925
number of thunks avoided: 1038428
number of attr lookups: 438419
number of primop calls: 474844
number of function calls: 959366
The one in Nixpkgs is O(n^2), this one is O(n). Big reduction in the
number of list allocations.
Statistics before (on a NixOS system config):
time elapsed: 1.17982
size of a value: 24
environments allocated: 1543334 (39624560 bytes)
list elements: 9612638 (76901104 bytes)
list concatenations: 37434
values allocated: 1854933 (44518392 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18272
number of thunks: 1392467
number of thunks avoided: 1507311
number of attr lookups: 430801
number of primop calls: 691600
number of function calls: 1492502
Statistics after:
time elapsed: 1.08683
size of a value: 24
environments allocated: 1384376 (35809568 bytes)
list elements: 6946783 (55574264 bytes)
list concatenations: 37434
values allocated: 1760440 (42250560 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18273
number of thunks: 1297673
number of thunks avoided: 1380759
number of attr lookups: 430802
number of primop calls: 628912
number of function calls: 1333544
Evaluation of a NixOS configuration spends quite a lot of time in the
"filter" function in Nixpkgs. As implemented in Nixpkgs, this is a
O(n^2) operation, so it's a good candidate for providing a more
efficient (i.e. primop) implementation. Using it gives a ~10% speed
increase and a significant reduction in the number of evaluations.
Statistics before (on a NixOS system config):
time elapsed: 1.3258
size of a value: 24
environments allocated: 1980939 (50127080 bytes)
list elements: 14679308 (117434464 bytes)
list concatenations: 50828
values allocated: 2098938 (50374512 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18271
number of thunks: 1645752
number of thunks avoided: 1921196
number of attr lookups: 430798
number of primop calls: 838807
number of function calls: 1930107
Statistics after:
time elapsed: 1.17982
size of a value: 24
environments allocated: 1543334 (39624560 bytes)
list elements: 9612638 (76901104 bytes)
list concatenations: 37434
values allocated: 1854933 (44518392 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18272
number of thunks: 1392467
number of thunks avoided: 1507311
number of attr lookups: 430801
number of primop calls: 691600
number of function calls: 1492502
Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator. It will count calls to functions
and primops as well as evaluations of attributes.
For example, to see where evaluation of a NixOS configuration spends
its time:
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
239532 head
233962 tail
191252 hasAttr
...
calls to 1595 functions:
224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
167377 undefined position
132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...
This is a problem because one process may set the immutable bit before
the second process has created its link.
Addressed random Hydra failures such as:
error: cannot rename `/nix/store/.tmp-link-17397-1804289383' to
`/nix/store/rsvzm574rlfip3830ac7kmaa028bzl6h-nixos-0.1pre-git/upstart-interface-version':
Operation not permitted
Channels are implemented using a profile now, and profiles contain a
manifest.nix file. This should be ignored to prevent bogus packages
from showing up in nix-env.
Since SubstitutionGoal::finished() in build.cc computes the hash
anyway, we can prevent the inefficiency of computing the hash twice by
letting the substituter tell Nix about the expected hash, which can
then verify it.
The generated attrset has drvPath and outPath with the right string context, type 'derivation', outputName with
the right name, all with a list of outputs, and an attribute for each output.
I see three uses for this (though certainly there may be more):
* Using derivations generated by something besides nix-instantiate (e.g. guix)
* Allowing packages provided by channels to be used in nix expressions. If a channel installed a valid deriver
for each package it provides into the store, then those could be imported and used as dependencies or installed
in environment.systemPackages, for example.
* Enable hydra to be consistent in how it treats inputs that are outputs of another build. Right now, if an
input is passed as an argument to the job, it is passed as a derivation, but if it is accessed via NIX_PATH
(i.e. through the <> syntax), then it is a path that can be imported. This is problematic because the build
being depended upon may have been built with non-obvious arguments passed to its jobset file. With this
feature, hydra can just set the name of that input to the path to its drv file in NIX_PATH
Incremental optimisation requires creating links in /nix/store/.links
to all files in the store. However, this means that if we delete a
store path, no files are actually deleted because links in
/nix/store/.links still exists. So we need to check /nix/store/.links
for files with a link count of 1 and delete them.
optimiseStore() now creates persistent, content-addressed hard links
in /nix/store/.links. For instance, if it encounters a file P with
hash H, it will create a hard link
P' = /nix/store/.link/<H>
to P if P' doesn't already exist; if P' exist, then P is replaced by a
hard link to P'. This is better than the previous in-memory map,
because it had the tendency to unnecessarily replace hard links with a
hard link to whatever happened to be the first file with a given hash
it encountered. It also allows on-the-fly, incremental optimisation.
To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5"). (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.) However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory. queryPathFromHashPart() prevents this by doing a cheap
database lookup.
queryValidPaths() combines multiple calls to isValidPath() in one.
This matters when using the Nix daemon because it reduces latency.
For instance, on "nix-env -qas \*" it reduces execution time from 5.7s
to 4.7s (which is indistinguishable from the non-daemon case).
Instead make a single call to querySubstitutablePathInfo() per
derivation output. This is faster and prevents having to implement
the "have" function in the binary cache substituter.
Getting substitute information using the binary cache substituter has
non-trivial latency overhead. A package or NixOS system configuration
can have hundreds of dependencies, and in the worst case (when the
local info cache is empty) we have to do a separate HTTP request for
each of these. If the ping time to the server is t, getting N info
files will take tN seconds; e.g., with a ping time of 0.1s to
nixos.org, sequentially downloading 1000 info files (a typical NixOS
config) will take at least 100 seconds.
To fix this problem, the binary cache substituter can now perform
requests in parallel. This required changing the substituter
interface to support a function querySubstitutablePathInfos() that
queries multiple paths at the same time, and rewriting queryMissing()
to take advantage of parallelism. (Due to local caching,
parallelising queryMissing() is sufficient for most use cases, since
it's almost always called before building a derivation and thus fills
the local info cache.)
For example, parallelism speeds up querying all 1056 paths in a
particular NixOS system configuration from 116s to 2.6s. It works so
well because the eccentricity of the top-level derivation in the
dependency graph is only 9. So we only need 10 round-trips (when
using an unlimited number of parallel connections) to get everything.
Currently we do a maximum of 150 parallel connections to the server.
Thus it's important that the binary cache server (e.g. nixos.org) has
a high connection limit. Alternatively we could use HTTP pipelining,
but WWW::Curl doesn't support it and libcurl has a hard-coded limit of
5 requests per pipeline.
In a private PID namespace, processes have PIDs that are separate from
the rest of the system. The initial child gets PID 1. Processes in
the chroot cannot see processes outside of the chroot. This improves
isolation between builds. However, processes on the outside can see
processes in the chroot and send signals to them (if they have
appropriate rights).
Since the builder gets PID 1, it serves as the reaper for zombies in
the chroot. This might turn out to be a problem. In that case we'll
need to have a small PID 1 process that sits in a loop calling wait().
In chroot builds, set the host name to "localhost" and the domain name
to "(none)" (the latter being the kernel's default). This improves
determinism a bit further.
P.S. I have to idea what UTS stands for.
This improves isolation a bit further, and it's just one extra flag in
the unshare() call.
P.S. It would be very cool to use CLONE_NEWPID (to put the builder in
a private PID namespace) as well, but that's slightly more risky since
having a builder start as PID 1 may cause problems.
On Linux it's possible to run a process in its own network namespace,
meaning that it gets its own set of network interfaces, disjunct from
the rest of the system. We use this to completely remove network
access to chroot builds, except that they get a private loopback
interface. This means that:
- Builders cannot connect to the outside network or to other processes
on the same machine, except processes within the same build.
- Vice versa, other processes cannot connect to processes in a chroot
build, and open ports/connections do not show up in "netstat".
- If two concurrent builders try to listen on the same port (e.g. as
part of a test), they no longer conflict with each other.
This was inspired by the "PrivateNetwork" flag in systemd.
Systemd can start the Nix daemon on demand when the Nix daemon socket
is first accessed. This is signalled through the LISTEN_FDS
environment variable, so all we need to do is check for that and then
use file descriptor 3 as the listen socket instead of creating one
ourselves.
