copyStorePath() now pipes the output of srcStore->narFromPath()
directly into dstStore->addToStore(). The sink used by the former is
converted into a source usable by the latter using
boost::coroutine2. This is based on [1].

This reduces the maximum resident size of

  $ nix build --store ~/my-nix/ /nix/store/b0zlxla7dmy1iwc3g459rjznx59797xy-binutils-2.28.1 --substituters file:///tmp/binary-cache-xz/ --no-require-sigs

from 418592 KiB to 53416 KiB. (The previous commit also reduced the
runtime from ~4.2s to ~3.4s, not sure why.) A further improvement will
be to download files into a Sink.

[1] https://github.com/NixOS/nix/compare/master...Mathnerd314:dump-fix-coroutine#diff-dcbcac55a634031f9cc73707da6e4b18

Issue #1969.

This allows decompression to happen in O(1) memory.

libexpr: Recognize newline in more places in lexer

The certificates won't get any better if we retry.

Fixes https://github.com/NixOS/nixpkgs/issues/37114.

Wrap thread local in function for Cygwin

Actually fixes #1976.

This is UB, even if the size is 0. See #1976.

Fixes #1976.

Note that this only requires headers from boost so it doesn't add a
runtime dependency.

Also, use Nixpkgs 18.03.

nix search: tests and fix #1893 and part of #1892

Fixes #1826. See #1352 for a previous instance of a similar change.

It was holding on to a Value* (i.e. a std::shared_ptr<ValidPathInfo>*)
outside of the pathInfoCache lock, so the std::shared_ptr could be
destroyed between the release of the lock and the decrement of the
std::shared_ptr refcount. This can happen if more than
'path-info-cache-size' paths are added in the meantime, *or* if
clearPathInfoCache() is called. The hydra-queue-runner queue monitor
thread periodically calls the later, so is likely to trigger a crash.

Fixes https://github.com/NixOS/hydra/issues/542.

Modified MakeBinOp to no longer produce its name using concatenation …

Doing so prevents emacs tags from working, as well as makes the code extremely
confusing for a newbie.

In the prior state, if someone wants to find the definition of "ExprApp" for
example, a grep through the code reveals nothing. Since the definition could be
hiding in numerous ".h" files, it's really difficult to find. This personally
took me several hours to figure out.

rand() -> random(), since we use srandom().

rand() requires we call srand() instead,
but might as well use random().

Move definition of destructor to logging.cc for
access to util.hh's ignoreException.

ssh-ng: Don't forward options to the daemon.

This can be iterated on and currently leaves out settings we know we
want to forward, but it fixes #1713 and fixes #1935 and isn't
fundamentally broken like the status quo. Future changes are suggested
in a comment.

Probably should point at the 18.03 release branch once that's made.

nix-channel grammar and punctuation

Flex's regexes have an annoying feature: the dot matches everything
except a newline. This causes problems for expressions like:

"${0}\
"

where the backslash-newline combination matches this rule instead of the
intended one mentioned in the comment:

    <STRING>\$|\\|\$\\ {
                    /* This can only occur when we reach EOF, otherwise the above
                    (...|\$[^\{\"\\]|\\.|\$\\.)+ would have triggered.
                    This is technically invalid, but we leave the problem to the
                    parser who fails with exact location. */
                    return STR;
                }
However, the parser actually accepts the resulting token sequence
('"' DOLLAR_CURLY 0 '}' STR '"'), which is a problem because the lexer
rule didn't assign anything to yylval. Ultimately this leads to a crash
when dereferencing a NULL pointer in ExprConcatStrings::bindVars().

The fix does change the syntax of the language in some corner cases
but I think it's only turning previously invalid (or crashing) syntax
to valid syntax. E.g.

"a\
b"

and

''a''\
b''

were previously syntax errors but now both result in "a\nb".

Found by afl-fuzz.

Minor changes to the nix-channel manpage for my first contribution

fetchGit: use "HEAD" as default ref instead of "master"

Without this the daemon won't be restarted if the process ever dies, for
example when sending a SIGHUP to reload nix.conf.

libexpr: Fix prim_replaceStrings() to work on an empty source string, take 2