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diff --git a/sub-process.h b/sub-process.h
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+#ifndef SUBPROCESS_H
+#define SUBPROCESS_H
+
+#include "git-compat-util.h"
+#include "hashmap.h"
+#include "run-command.h"
+
+/*
+ * The sub-process API makes it possible to run background sub-processes
+ * for the entire lifetime of a Git invocation. If Git needs to communicate
+ * with an external process multiple times, then this can reduces the process
+ * invocation overhead. Git and the sub-process communicate through stdin and
+ * stdout.
+ *
+ * The sub-processes are kept in a hashmap by command name and looked up
+ * via the subprocess_find_entry function.  If an existing instance can not
+ * be found then a new process should be created and started.  When the
+ * parent git command terminates, all sub-processes are also terminated.
+ *
+ * This API is based on the run-command API.
+ */
+
+ /* data structures */
+
+/* Members should not be accessed directly. */
+struct subprocess_entry {
+	struct hashmap_entry ent; /* must be the first member! */
+	const char *cmd;
+	struct child_process process;
+};
+
+struct subprocess_capability {
+	const char *name;
+
+	/*
+	 * subprocess_handshake will "|=" this value to supported_capabilities
+	 * if the server reports that it supports this capability.
+	 */
+	unsigned int flag;
+};
+
+/* subprocess functions */
+
+/* Function to test two subprocess hashmap entries for equality. */
+extern int cmd2process_cmp(const void *unused_cmp_data,
+			   const void *e1,
+			   const void *e2,
+			   const void *unused_keydata);
+
+/*
+ * User-supplied function to initialize the sub-process.  This is
+ * typically used to negotiate the interface version and capabilities.
+ */
+typedef int(*subprocess_start_fn)(struct subprocess_entry *entry);
+
+/* Start a subprocess and add it to the subprocess hashmap. */
+int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, const char *cmd,
+		subprocess_start_fn startfn);
+
+/* Kill a subprocess and remove it from the subprocess hashmap. */
+void subprocess_stop(struct hashmap *hashmap, struct subprocess_entry *entry);
+
+/* Find a subprocess in the subprocess hashmap. */
+struct subprocess_entry *subprocess_find_entry(struct hashmap *hashmap, const char *cmd);
+
+/* subprocess helper functions */
+
+/* Get the underlying `struct child_process` from a subprocess. */
+static inline struct child_process *subprocess_get_child_process(
+		struct subprocess_entry *entry)
+{
+	return &entry->process;
+}
+
+/*
+ * Perform the version and capability negotiation as described in the "Long
+ * Running Filter Process" section of the gitattributes documentation using the
+ * given requested versions and capabilities. The "versions" and "capabilities"
+ * parameters are arrays terminated by a 0 or blank struct.
+ *
+ * This function is typically called when a subprocess is started (as part of
+ * the "startfn" passed to subprocess_start).
+ */
+int subprocess_handshake(struct subprocess_entry *entry,
+			 const char *welcome_prefix,
+			 int *versions,
+			 int *chosen_version,
+			 struct subprocess_capability *capabilities,
+			 unsigned int *supported_capabilities);
+
+/*
+ * Helper function that will read packets looking for "status=<foo>"
+ * key/value pairs and return the value from the last "status" packet
+ */
+
+int subprocess_read_status(int fd, struct strbuf *status);
+
+#endif