Fix #1762

nix-store --export, nix-store --dump, and nix dump-path would previously
fail silently if writing the data out failed, because
 a) FdSink::write ignored exceptions, and
 b) the commands relied on FdSink's destructor, which ignores
    exceptions, to flush the data out.

This could cause rather opaque issues with installing nixos, because
nix-store --export would happily proceed even if it couldn't write its
data out (e.g. if nix-store --import on the other side of the pipe
failed).

This commit adds tests that expose these issues in the nix-store
commands, and fixes them for all three.

tests/export.sh

@@ -8,6 +8,11 @@ nix-store --export $outPath > $TEST_ROOT/exp
 
 nix-store --export $(nix-store -qR $outPath) > $TEST_ROOT/exp_all
 
+if nix-store --export $outPath >/dev/full ; then
+    echo "exporting to a bad file descriptor should fail"
+    exit 1
+fi
+
 
 clearStore
 

tests/nar-access.sh

@@ -36,3 +36,9 @@ diff -u baz.cat-nar $storePath/foo/baz
 # Test missing files.
 nix ls-store --json -R $storePath/xyzzy 2>&1 | grep 'does not exist in NAR'
 nix ls-store $storePath/xyzzy 2>&1 | grep 'does not exist'
+
+# Test failure to dump.
+if nix-store --dump $storePath >/dev/full ; then
+    echo "dumping to /dev/full should fail"
+    exit -1
+fi