Allow extensions to add new backup targets.

Commit 3500ccc39b0dadd1068a03938e4b8ff562587ccc allowed for base backup
targets, meaning that we could do something with the backup other than
send it to the client, but all of those targets had to be baked in to
the core code. This commit makes it possible for extensions to define
additional backup targets.

Patch by me, reviewed by Abhijit Menon-Sen.

Discussion: http://postgr.es/m/CA+TgmoaqvdT-u3nt+_kkZ7bgDAyqDB0i-+XOMmr5JN2Rd37hxw@mail.gmail.com

1 files changed, 238 insertions, 0 deletions

diff --git a/src/backend/replication/basebackup_target.c b/src/backend/replication/basebackup_target.c
new file mode 100644
index 00000000000..d93f5e02dbb
--- /dev/null
+++ b/src/backend/replication/basebackup_target.c
@@ -0,0 +1,238 @@
+/*-------------------------------------------------------------------------
+ *
+ * basebackup_target.c
+ *	  Base backups can be "targetted," which means that they can be sent
+ *	  somewhere other than to the client which requested the backup.
+ *	  Furthermore, new targets can be defined by extensions. This file
+ *	  contains code to support that functionality.
+ *
+ * Portions Copyright (c) 2010-2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/replication/basebackup_gzip.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "replication/basebackup_target.h"
+#include "utils/memutils.h"
+
+typedef struct BaseBackupTargetType
+{
+	char	   *name;
+	void	   *(*check_detail) (char *, char *);
+	bbsink	   *(*get_sink) (bbsink *, void *);
+} BaseBackupTargetType;
+
+struct BaseBackupTargetHandle
+{
+	BaseBackupTargetType *type;
+	void	   *detail_arg;
+};
+
+static void initialize_target_list(void);
+extern bbsink *blackhole_get_sink(bbsink *next_sink, void *detail_arg);
+extern bbsink *server_get_sink(bbsink *next_sink, void *detail_arg);
+static void *reject_target_detail(char *target, char *target_detail);
+static void *server_check_detail(char *target, char *target_detail);
+
+static BaseBackupTargetType builtin_backup_targets[] =
+{
+	{
+		"blackhole", reject_target_detail, blackhole_get_sink
+	},
+	{
+		"server", server_check_detail, server_get_sink
+	},
+	{
+		NULL
+	}
+};
+
+static List *BaseBackupTargetTypeList = NIL;
+
+/*
+ * Add a new base backup target type.
+ *
+ * This is intended for use by server extensions.
+ */
+void
+BaseBackupAddTarget(char *name,
+					void *(*check_detail) (char *, char *),
+					bbsink *(*get_sink) (bbsink *, void *))
+{
+	BaseBackupTargetType *ttype;
+	MemoryContext oldcontext;
+	ListCell   *lc;
+
+	/* If the target list is not yet initialized, do that first. */
+	if (BaseBackupTargetTypeList == NIL)
+		initialize_target_list();
+
+	/* Search the target type list for an existing entry with this name. */
+	foreach(lc, BaseBackupTargetTypeList)
+	{
+		BaseBackupTargetType *ttype = lfirst(lc);
+
+		if (strcmp(ttype->name, name) == 0)
+		{
+			/*
+			 * We found one, so update it.
+			 *
+			 * It is probably not a great idea to call BaseBackupAddTarget
+			 * for the same name multiple times, but if it happens, this
+			 * seems like the sanest behavior.
+			 */
+			ttype->check_detail = check_detail;
+			ttype->get_sink = get_sink;
+			return;
+		}
+	}
+
+	/*
+	 * We use TopMemoryContext for allocations here to make sure that the
+	 * data we need doesn't vanish under us; that's also why we copy the
+	 * target name into a newly-allocated chunk of memory.
+	 */
+	oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+	ttype = palloc(sizeof(BaseBackupTargetType));
+	ttype->name = pstrdup(name);
+	ttype->check_detail = check_detail;
+	ttype->get_sink = get_sink;
+	BaseBackupTargetTypeList = lappend(BaseBackupTargetTypeList, ttype);
+	MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Look up a base backup target and validate the target_detail.
+ *
+ * Extensions that define new backup targets will probably define a new
+ * type of bbsink to match. Validation of the target_detail can be performed
+ * either in the check_detail routine called here, or in the bbsink
+ * constructor, which will be called from BaseBackupGetSink. It's mostly
+ * a matter of taste, but the check_detail function runs somewhat earlier.
+ */
+BaseBackupTargetHandle *
+BaseBackupGetTargetHandle(char *target, char *target_detail)
+{
+	ListCell   *lc;
+
+	/* If the target list is not yet initialized, do that first. */
+	if (BaseBackupTargetTypeList == NIL)
+		initialize_target_list();
+
+	/* Search the target type list for a match. */
+	foreach(lc, BaseBackupTargetTypeList)
+	{
+		BaseBackupTargetType *ttype = lfirst(lc);
+
+		if (strcmp(ttype->name, target) == 0)
+		{
+			BaseBackupTargetHandle *handle;
+
+			/* Found the target. */
+			handle = palloc(sizeof(BaseBackupTargetHandle));
+			handle->type = ttype;
+			handle->detail_arg = ttype->check_detail(target, target_detail);
+
+			return handle;
+		}
+	}
+
+	/* Did not find the target. */
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("unrecognized target: \"%s\"", target)));
+}
+
+/*
+ * Construct a bbsink that will implement the backup target.
+ *
+ * The get_sink function does all the real work, so all we have to do here
+ * is call it with the correct arguments. Whatever the check_detail function
+ * returned is here passed through to the get_sink function. This lets those
+ * two functions communicate with each other, if they wish. If not, the
+ * check_detail function can simply return the target_detail and let the
+ * get_sink function take it from there.
+ */
+bbsink *
+BaseBackupGetSink(BaseBackupTargetHandle *handle, bbsink *next_sink)
+{
+	return handle->type->get_sink(next_sink, handle->detail_arg);
+}
+
+/*
+ * Load predefined target types into BaseBackupTargetTypeList.
+ */
+static void
+initialize_target_list(void)
+{
+	BaseBackupTargetType *ttype = builtin_backup_targets;
+	MemoryContext oldcontext;
+
+	oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+	while (ttype->name != NULL)
+	{
+		BaseBackupTargetTypeList = lappend(BaseBackupTargetTypeList, ttype);
+		++ttype;
+	}
+	MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Normally, a get_sink function should construct and return a new bbsink that
+ * implements the backup target, but the 'blackhole' target just throws the
+ * data away. We could implement that by adding a bbsink that does nothing
+ * but forward, but it's even cheaper to implement that by not adding a bbsink
+ * at all.
+ */
+bbsink *
+blackhole_get_sink(bbsink *next_sink, void *detail_arg)
+{
+	return next_sink;
+}
+
+/*
+ * Create a bbsink implementing a server-side backup.
+ */
+bbsink *
+server_get_sink(bbsink *next_sink, void *detail_arg)
+{
+	return bbsink_server_new(next_sink, detail_arg);
+}
+
+/*
+ * Implement target-detail checking for a target that does not accept a
+ * detail.
+ */
+void *
+reject_target_detail(char *target, char *target_detail)
+{
+	if (target_detail != NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("target '%s' does not accept a target detail",
+						target)));
+
+	return NULL;
+}
+
+/*
+ * Implement target-detail checking for a server-side backup.
+ *
+ * target_detail should be the name of the directory to which the backup
+ * should be written, but we don't check that here. Rather, that check,
+ * as well as the necessary permissions checking, happens in bbsink_server_new.
+ */
+void *
+server_check_detail(char *target, char *target_detail)
+{
+	if (target_detail == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("target '%s' requires a target detail",
+						target)));
+
+	return target_detail;
+}