add osmo_use_count API

Provide a common implementation of use counting that supports naming each user
as well as counting more than just one use per user, depending on the rules the
caller implies.

In osmo-msc, we were originally using a simple int counter to see whether a
connection is still in use or should be discarded. For clarity, we later added
names to each user in the form of a bitmask of flags, to figure out exactly
which users are still active: for logging and to debug double get / double put
bugs. This however is still not adequate, since there may be more than one CM
Service Request pending. Also, it is a specialized implementation that is not
re-usable.

With this generalized implementation, we can:

- fix the problem of inadequate counting of multiple concurrent CM Service
  Requests (more than one use count per user category),
- directly use arbitrary names for uses like __func__ or "foo" (no need to
  define enums and value_string[]s),
- re-use the same code for e.g. vlr_subscr and get fairly detailed VLR
  susbscriber usage logging for free.

Change-Id: Ife31e6798b4e728a23913179e346552a7dd338c0

2 files changed, 280 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 1fae8b0a..aaf2233e 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -26,6 +26,7 @@ libosmocore_la_SOURCES = timer.c timer_gettimeofday.c timer_clockgettime.c \
 			 isdnhdlc.c \
 			 tdef.c \
 			 sockaddr_str.c \
+			 use_count.c \
 			 $(NULL)
 
 if HAVE_SSSE3
diff --git a/src/use_count.c b/src/use_count.c
new file mode 100644
index 00000000..453d2ae8
--- /dev/null
+++ b/src/use_count.c
@@ -0,0 +1,279 @@
+/*! \file use_count.c
+ * Generic object usage counter Implementation (get, put and deallocate on zero count).
+ */
+/*
+ * (C) 2019 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <errno.h>
+#include <inttypes.h>
+#include <string.h>
+
+#include <osmocom/core/linuxlist.h>
+#include <osmocom/core/utils.h>
+#include <osmocom/core/use_count.h>
+
+/*! \addtogroup use_count
+ *
+ * Generic object usage counter (get, put and deallocate on zero count).
+ *
+ * For an example and a detailed description, see struct osmo_use_count.
+ *
+ * @{
+ * \file use_count.c
+ */
+
+/*! Add two int32_t but make sure to min- and max-clamp at INT32_MIN and INT32_MAX, respectively. */
+static inline bool count_safe(int32_t *val_p, int32_t add)
+{
+	int32_t val = *val_p;
+
+	/* A simpler implementation would just let the integer overflow and compare with previous value afterwards, but
+	 * that causes runtime errors in the address sanitizer. So let's just do this without tricks. */
+	if (add < 0 && val < 0 && val - INT32_MIN < -add) {
+		*val_p = INT32_MIN;
+		return false;
+	}
+
+	if (add > 0 && val > 0 && INT32_MAX - val < add) {
+		*val_p = INT32_MAX;
+		return false;
+	}
+
+	*val_p = val + add;
+	return true;
+}
+
+/*! Return the sum of all use counts, min- and max-clamped at INT32_MIN and INT32_MAX.
+ * \param[in] uc  Use counts to sum up.
+ * \return Accumulated counts, or 0 if uc is NULL.
+ */
+int32_t osmo_use_count_total(const struct osmo_use_count *uc)
+{
+	struct osmo_use_count_entry *e;
+	int32_t total = 0;
+
+	if (!uc || !uc->use_counts.next)
+		return 0;
+
+	llist_for_each_entry(e, &uc->use_counts, entry) {
+		count_safe(&total, e->count);
+	}
+	return total;
+}
+
+/*! Return use count by a single use token.
+ * \param[in] uc  Use counts to look up in.
+ * \param[in] use  Use token.
+ * \return Use count, or 0 if uc is NULL or use token is not present.
+ */
+int32_t osmo_use_count_by(const struct osmo_use_count *uc, const char *use)
+{
+	const struct osmo_use_count_entry *e;
+	if (!uc)
+		return 0;
+	e = osmo_use_count_find(uc, use);
+	if (!e)
+		return 0;
+	return e->count;
+}
+
+/*! Write a comprehensive listing of use counts to a string buffer.
+ * Reads like "12 (3*barring,fighting,8*kungfoo)".
+ * \param[inout] buf  Destination buffer.
+ * \param[in] buf_len  sizeof(buf).
+ * \param[in] uc  Use counts to print.
+ * \return buf, always nul-terminated (except when buf_len < 1).
+ */
+const char *osmo_use_count_name_buf(char *buf, size_t buf_len, const struct osmo_use_count *uc)
+{
+	int32_t count = osmo_use_count_total(uc);
+	struct osmo_strbuf sb = { .buf = buf, .len = buf_len };
+	struct osmo_use_count_entry *e;
+	bool first;
+
+	OSMO_STRBUF_PRINTF(sb, "%" PRId32 " (", count);
+
+	first = true;
+	llist_for_each_entry(e, &uc->use_counts, entry) {
+		if (!e->count)
+			continue;
+		if (!first)
+			OSMO_STRBUF_PRINTF(sb, ",");
+		first = false;
+		if (e->count != 1)
+			OSMO_STRBUF_PRINTF(sb, "%" PRId32 "*", e->count);
+		OSMO_STRBUF_PRINTF(sb, "%s", e->use ? : "NULL");
+	}
+	if (first)
+		OSMO_STRBUF_PRINTF(sb, "-");
+	OSMO_STRBUF_PRINTF(sb, ")");
+	return buf;
+}
+
+/* Return a use token's use count entry -- probably you want osmo_use_count_by() instead.
+ * \param[in] uc  Use counts to look up in.
+ * \param[in] use  Use token.
+ * \return matching entry, or NULL if not present.
+ */
+struct osmo_use_count_entry *osmo_use_count_find(const struct osmo_use_count *uc, const char *use)
+{
+	struct osmo_use_count_entry *e;
+	if (!uc->use_counts.next)
+		return NULL;
+	llist_for_each_entry(e, &uc->use_counts, entry) {
+		if (e->use == use || (use && e->use && !strcmp(e->use, use)))
+			return e;
+	}
+	return NULL;
+}
+
+/*! Find a use count entry that currently has zero count, and re-use that for this new use token. */
+static struct osmo_use_count_entry *osmo_use_count_repurpose_zero_entry(struct osmo_use_count *uc, const char *use)
+{
+	struct osmo_use_count_entry *e;
+	if (!uc->use_counts.next)
+		return NULL;
+	llist_for_each_entry(e, &uc->use_counts, entry) {
+		if (!e->count) {
+			e->use = use;
+			return e;
+		}
+	}
+	return NULL;
+}
+
+/*! Allocate a new use count entry, happens implicitly in osmo_use_count_get_put(). */
+static struct osmo_use_count_entry *osmo_use_count_create(struct osmo_use_count *uc, const char *use)
+{
+	struct osmo_use_count_entry *e = talloc_zero(uc->talloc_object, struct osmo_use_count_entry);
+	if (!e)
+		return NULL;
+	*e = (struct osmo_use_count_entry){
+		.use_count = uc,
+		.use = use,
+	};
+	if (!uc->use_counts.next)
+		INIT_LLIST_HEAD(&uc->use_counts);
+	llist_add_tail(&e->entry, &uc->use_counts);
+	return e;
+}
+
+/*! Deallocate a use count entry.
+ * Normally, this is not necessary -- it is ok and even desirable to leave use count entries around even when they reach
+ * a count of zero, until the use_count->talloc_object deallocates and removes all of them in one flush. This avoids
+ * repeated allocation and deallocation for use tokens, because use count entries that have reached zero count are
+ * repurposed for any other use tokens. A cleanup makes sense only if a very large number of differing use tokens surged
+ * at the same time, and the owning object will not be deallocated soon; if so, this should be done by the
+ * osmo_use_count_cb_t implementation.
+ *
+ * osmo_use_count_free() must *not* be called on use count entries that were added by
+ * osmo_use_count_make_static_entries(). This is the responsibility of the osmo_use_count_cb_t() implementation.
+ *
+ * \param[in] use_count_entry  Use count entry to unlist and free.
+ */
+void osmo_use_count_free(struct osmo_use_count_entry *use_count_entry)
+{
+	if (!use_count_entry)
+		return;
+	llist_del(&use_count_entry->entry);
+	talloc_free(use_count_entry);
+}
+
+/*! Implementation for osmo_use_count_get_put(), which can also be directly invoked to pass source file information. For
+ * arguments besides file and line, see osmo_use_count_get_put().
+ * \param[in] file  Source file path, as in __FILE__.
+ * \param[in] line  Source file line, as in __LINE__.
+ */
+int _osmo_use_count_get_put(struct osmo_use_count *uc, const char *use, int32_t change,
+			    const char *file, int line)
+{
+	struct osmo_use_count_entry *e;
+	int32_t old_use_count;
+	if (!change)
+		return 0;
+
+	e = osmo_use_count_find(uc, use);
+	if (!e)
+		e = osmo_use_count_repurpose_zero_entry(uc, use);
+	if (!e)
+		e = osmo_use_count_create(uc, use);
+	if (!e)
+		return -ENOMEM;
+
+	if (!e->count) {
+		/* move to end */
+		llist_del(&e->entry);
+		llist_add_tail(&e->entry, &uc->use_counts);
+	}
+
+	old_use_count = e->count;
+	if (!count_safe(&e->count, change)) {
+		e->count = old_use_count;
+		return -ERANGE;
+	}
+
+	if (uc->use_cb)
+		return uc->use_cb(e, old_use_count, file, line);
+	return 0;
+}
+
+/*! Add N static use token entries to avoid dynamic allocation of use count tokens.
+ * When not using this function, use count entries are talloc allocated from uc->talloc_object as talloc context. This
+ * means that there are small dynamic allocations for each use count token. osmo_use_count_get_put() normally leaves
+ * zero-count entries around and re-purposes them later, so the number of small allocations is at most the number of
+ * concurrent differently-named uses of the same object. If that is not enough, this function allows completely avoiding
+ * dynamic use count allocations, by adding N static entries with a zero count and a NULL use token.  They will be used
+ * by osmo_use_count_get_put(), and, if the caller avoids using osmo_use_count_free(), the osmo_use_count implementation
+ * never deallocates them. The idea is that the entries are members of the uc->talloc_object, or that they will by other
+ * means be implicitly deallocated by the talloc_object. It is fine to call
+ * osmo_use_count_make_static_entries(buf_n_entries=N) and later have more than N concurrent uses, i.e. it is no problem
+ * to mix static and dynamic entries. To completely avoid dynamic use count entries, N has to >= the maximum number of
+ * concurrent differently-named uses that will occur in the lifetime of the talloc_object.
+ *
+ *    struct my_object {
+ *            struct osmo_use_count use_count;
+ *            struct osmo_use_count_entry use_count_buf[3]; // planning for 3 concurrent users
+ *    };
+ *
+ *    void example() {
+ *            struct my_object *o = talloc_zero(ctx, struct my_object);
+ *            osmo_use_count_make_static_entries(&o->use_count, o->use_count_buf, ARRAY_SIZE(o->use_count_buf));
+ *    }
+ */
+void osmo_use_count_make_static_entries(struct osmo_use_count *uc, struct osmo_use_count_entry *buf,
+					size_t buf_n_entries)
+{
+	size_t idx;
+	if (!uc->use_counts.next)
+		INIT_LLIST_HEAD(&uc->use_counts);
+	for (idx = 0; idx < buf_n_entries; idx++) {
+		struct osmo_use_count_entry *e = &buf[idx];
+		*e = (struct osmo_use_count_entry){
+			.use_count = uc,
+		};
+		llist_add_tail(&e->entry, &uc->use_counts);
+	}
+}
+
+/*! @} */