Add reftable library

Reftable is a new format for storing the ref database. It provides the
following benefits:

 * Simple and fast atomic ref transactions, including multiple refs and reflogs.
 * Compact storage of ref data.
 * Fast look ups of ref data.
 * Case-sensitive ref names on Windows/OSX, regardless of file system
 * Eliminates file/directory conflicts in ref names

Further context and motivation can be found in background reading:

* Spec: https://github.com/eclipse/jgit/blob/master/Documentation/technical/reftable.md

* Original discussion on JGit-dev:  https://www.eclipse.org/lists/jgit-dev/msg03389.html

* First design discussion on git@vger: https://public-inbox.org/git/CAJo=hJtTp2eA3z9wW9cHo-nA7kK40vVThqh6inXpbCcqfdMP9g@mail.gmail.com/

* Last design discussion on git@vger: https://public-inbox.org/git/CAJo=hJsZcAM9sipdVr7TMD-FD2V2W6_pvMQ791EGCDsDkQ033w@mail.gmail.com/

* First attempt at implementation: https://public-inbox.org/git/CAP8UFD0PPZSjBnxCA7ez91vBuatcHKQ+JUWvTD1iHcXzPBjPBg@mail.gmail.com/

* libgit2 support issue: https://github.com/libgit2/libgit2/issues

* GitLab support issue: https://gitlab.com/gitlab-org/git/issues/6

* go-git support issue: src-d/go-git#1059

Signed-off-by: Han-Wen Nienhuys <[email protected]>

reftable/LICENSE

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+BSD License
+
+Copyright (c) 2020, Google LLC
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+* Redistributions of source code must retain the above copyright notice,
+this list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright
+notice, this list of conditions and the following disclaimer in the
+documentation and/or other materials provided with the distribution.
+
+* Neither the name of Google LLC nor the names of its contributors may
+be used to endorse or promote products derived from this software
+without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

reftable/README.md

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+
+The source code in this directory comes from https://github.com/google/reftable.
+
+The VERSION file keeps track of the current version of the reftable library.
+
+To update the library, do:
+
+   ((cd reftable-repo && git fetch origin && git checkout origin/master ) ||
+    git clone https://github.com/google/reftable reftable-repo) && \
+   cp reftable-repo/c/*.[ch] reftable/ && \
+   cp reftable-repo/LICENSE reftable/ &&
+   git --git-dir reftable-repo/.git show --no-patch origin/master \
+    > reftable/VERSION && \
+   echo '/* empty */' > reftable/config.h
+   rm reftable/*_test.c reftable/test_framework.*
+   git add reftable/*.[ch]
+
+Bugfixes should be accompanied by a test and applied to upstream project at
+https://github.com/google/reftable.

reftable/VERSION

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+commit 6115b50fdb9bc662be39b05f5589bc109282ae7f
+Author: Han-Wen Nienhuys <[email protected]>
+Date:   Mon Feb 10 13:59:52 2020 +0100
+
+    README: add a note about the Java implementation

reftable/basics.c

@@ -0,0 +1,196 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "basics.h"
+
+#include "system.h"
+
+void put_u24(byte *out, uint32_t i)
+{
+	out[0] = (byte)((i >> 16) & 0xff);
+	out[1] = (byte)((i >> 8) & 0xff);
+	out[2] = (byte)((i)&0xff);
+}
+
+uint32_t get_u24(byte *in)
+{
+	return (uint32_t)(in[0]) << 16 | (uint32_t)(in[1]) << 8 |
+	       (uint32_t)(in[2]);
+}
+
+void put_u32(byte *out, uint32_t i)
+{
+	out[0] = (byte)((i >> 24) & 0xff);
+	out[1] = (byte)((i >> 16) & 0xff);
+	out[2] = (byte)((i >> 8) & 0xff);
+	out[3] = (byte)((i)&0xff);
+}
+
+uint32_t get_u32(byte *in)
+{
+	return (uint32_t)(in[0]) << 24 | (uint32_t)(in[1]) << 16 |
+	       (uint32_t)(in[2]) << 8 | (uint32_t)(in[3]);
+}
+
+void put_u64(byte *out, uint64_t v)
+{
+	int i = 0;
+	for (i = sizeof(uint64_t); i--;) {
+		out[i] = (byte)(v & 0xff);
+		v >>= 8;
+	}
+}
+
+uint64_t get_u64(byte *out)
+{
+	uint64_t v = 0;
+	int i = 0;
+	for (i = 0; i < sizeof(uint64_t); i++) {
+		v = (v << 8) | (byte)(out[i] & 0xff);
+	}
+	return v;
+}
+
+void put_u16(byte *out, uint16_t i)
+{
+	out[0] = (byte)((i >> 8) & 0xff);
+	out[1] = (byte)((i)&0xff);
+}
+
+uint16_t get_u16(byte *in)
+{
+	return (uint32_t)(in[0]) << 8 | (uint32_t)(in[1]);
+}
+
+/*
+  find smallest index i in [0, sz) at which f(i) is true, assuming
+  that f is ascending. Return sz if f(i) is false for all indices.
+*/
+int binsearch(int sz, int (*f)(int k, void *args), void *args)
+{
+	int lo = 0;
+	int hi = sz;
+
+	/* invariant: (hi == sz) || f(hi) == true
+	   (lo == 0 && f(0) == true) || fi(lo) == false
+	 */
+	while (hi - lo > 1) {
+		int mid = lo + (hi - lo) / 2;
+
+		int val = f(mid, args);
+		if (val) {
+			hi = mid;
+		} else {
+			lo = mid;
+		}
+	}
+
+	if (lo == 0) {
+		if (f(0, args)) {
+			return 0;
+		} else {
+			return 1;
+		}
+	}
+
+	return hi;
+}
+
+void free_names(char **a)
+{
+	char **p = a;
+	if (p == NULL) {
+		return;
+	}
+	while (*p) {
+		free(*p);
+		p++;
+	}
+	free(a);
+}
+
+int names_length(char **names)
+{
+	int len = 0;
+	for (char **p = names; *p; p++) {
+		len++;
+	}
+	return len;
+}
+
+/* parse a newline separated list of names. Empty names are discarded. */
+void parse_names(char *buf, int size, char ***namesp)
+{
+	char **names = NULL;
+	int names_cap = 0;
+	int names_len = 0;
+
+	char *p = buf;
+	char *end = buf + size;
+	while (p < end) {
+		char *next = strchr(p, '\n');
+		if (next != NULL) {
+			*next = 0;
+		} else {
+			next = end;
+		}
+		if (p < next) {
+			if (names_len == names_cap) {
+				names_cap = 2 * names_cap + 1;
+				names = realloc(names,
+						names_cap * sizeof(char *));
+			}
+			names[names_len++] = strdup(p);
+		}
+		p = next + 1;
+	}
+
+	if (names_len == names_cap) {
+		names_cap = 2 * names_cap + 1;
+		names = realloc(names, names_cap * sizeof(char *));
+	}
+
+	names[names_len] = NULL;
+	*namesp = names;
+}
+
+int names_equal(char **a, char **b)
+{
+	while (*a && *b) {
+		if (strcmp(*a, *b)) {
+			return 0;
+		}
+
+		a++;
+		b++;
+	}
+
+	return *a == *b;
+}
+
+const char *error_str(int err)
+{
+	switch (err) {
+	case IO_ERROR:
+		return "I/O error";
+	case FORMAT_ERROR:
+		return "FORMAT_ERROR";
+	case NOT_EXIST_ERROR:
+		return "NOT_EXIST_ERROR";
+	case LOCK_ERROR:
+		return "LOCK_ERROR";
+	case API_ERROR:
+		return "API_ERROR";
+	case ZLIB_ERROR:
+		return "ZLIB_ERROR";
+	case -1:
+		return "general error";
+	default:
+		return "unknown error code";
+	}
+}

reftable/basics.h

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+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef BASICS_H
+#define BASICS_H
+
+#include "system.h"
+
+#include "reftable.h"
+
+#define true 1
+#define false 0
+
+void put_u24(byte *out, uint32_t i);
+uint32_t get_u24(byte *in);
+
+uint64_t get_u64(byte *in);
+void put_u64(byte *out, uint64_t i);
+
+void put_u32(byte *out, uint32_t i);
+uint32_t get_u32(byte *in);
+
+void put_u16(byte *out, uint16_t i);
+uint16_t get_u16(byte *in);
+int binsearch(int sz, int (*f)(int k, void *args), void *args);
+
+void free_names(char **a);
+void parse_names(char *buf, int size, char ***namesp);
+int names_equal(char **a, char **b);
+int names_length(char **names);
+
+#endif