Fix memory leaks in dmu_send()/dmu_send_obj()

[ryao]

Motivation and Context

If we encounter an EXDEV error when using the redacted snapshots feature, the memory used by dspp.fromredactsnaps is leaked.

Clang's static analyzer caught this during an experiment in which I had annotated various headers in an attempt to improve the results of static analysis.

Description

We add a check to free the memory so that it is not leaked.

How Has This Been Tested?

A build test has been done.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ryao]

As a note to reviewers, look for SET_ERROR(EXDEV) in the code. There is another EXDEV in dmu_send() that might initially make it look like a false positive. The real one that causes the leak is before that.

[ryao]

For the curious, the following experimental change to our header files enables Clang's static analyzer to find this:

diff --git a/include/sys/zfs_context.h b/include/sys/zfs_context.h
index d29d7118f..a439b6903 100644
--- a/include/sys/zfs_context.h
+++ b/include/sys/zfs_context.h
@@ -403,6 +403,33 @@ void procfs_list_destroy(procfs_list_t *procfs_list);
 void procfs_list_add(procfs_list_t *procfs_list, void *p);
 #endif
 
+#ifdef DEBUG
+
+#include <umem.h>
+
+__attribute__((returns_nonnull, alloc_size(1), malloc))
+static inline void *
+umem_alloc_nofail(size_t size, int kmflags)
+{
+	return (umem_alloc(size, kmflags));
+}
+
+__attribute__((returns_nonnull, alloc_size(1), malloc))
+static inline void *
+umem_zalloc_nofail(size_t size, int kmflags)
+{
+	return (umem_alloc(size, kmflags));
+}
+
+__attribute__((returns_nonnull, malloc))
+static inline void *
+umem_cache_alloc_nofail(umem_cache_t *cp, int flags)
+{
+	return (umem_cache_alloc(cp, flags));
+}
+
+#endif /* DEBUG */
+
 /*
  * Kernel memory
  */
@@ -412,8 +439,15 @@ void procfs_list_add(procfs_list_t *procfs_list, void *p);
 #define	KM_NORMALPRI		0	/* not needed with UMEM_DEFAULT */
 #define	KMC_NODEBUG		UMC_NODEBUG
 #define	KMC_KVMEM		0x0
-#define	kmem_alloc(_s, _f)	umem_alloc(_s, _f)
-#define	kmem_zalloc(_s, _f)	umem_zalloc(_s, _f)
+#ifdef DEBUG
+#define	kmem_alloc(_s, _f)	(((_f) == KM_SLEEP) ?			\
+	    umem_alloc_nofail(_s, _f) : umem_alloc(_s, _f))
+#define	kmem_zalloc(_s, _f)	(((_f) == KM_SLEEP) ?			\
+	    umem_zalloc_nofail(_s, _f) : umem_zalloc(_s, _f))
+#else
+#define	kmem_alloc(_b, _s)	umem_alloc(_b, _s)
+#define	kmem_zalloc(_b, _s)	umem_zalloc(_b, _s)
+#endif /* DEBUG */
 #define	kmem_free(_b, _s)	umem_free(_b, _s)
 #define	vmem_alloc(_s, _f)	kmem_alloc(_s, _f)
 #define	vmem_zalloc(_s, _f)	kmem_zalloc(_s, _f)
@@ -421,7 +455,12 @@ void procfs_list_add(procfs_list_t *procfs_list, void *p);
 #define	kmem_cache_create(_a, _b, _c, _d, _e, _f, _g, _h, _i) \
 	umem_cache_create(_a, _b, _c, _d, _e, _f, _g, _h, _i)
 #define	kmem_cache_destroy(_c)	umem_cache_destroy(_c)
+#ifdef DEBUG
+#define	kmem_cache_alloc(_s, _f)	(((_f) == KM_SLEEP) ?		\
+	    umem_cache_alloc_nofail(_s, _f) : umem_cache_alloc(_s, _f))
+#else
 #define	kmem_cache_alloc(_c, _f) umem_cache_alloc(_c, _f)
+#endif /* DEBUG */
 #define	kmem_cache_free(_c, _b)	umem_cache_free(_c, _b)
 #define	kmem_debugging()	0
 #define	kmem_cache_reap_now(_c)	umem_cache_reap_now(_c);