Merge remote-tracking branch 'remotes/qmp-unstable/queue/qmp' into staging

* remotes/qmp-unstable/queue/qmp: (38 commits)
  Revert "qapi: Clean up superfluous null check in qapi_dealloc_type_str()"
  qapi: Document optional arguments' backwards compatibility
  qmp: use valid JSON in transaction example
  qmp: Don't use error_is_set() to suppress additional errors
  dump: Drop pointless error_is_set(), DumpState member errp
  qemu-option: Clean up fragile use of error_is_set()
  qga: Drop superfluous error_is_set()
  qga: Clean up fragile use of error_is_set()
  qapi: Clean up fragile use of error_is_set()
  tests/qapi-schema: Drop superfluous error_is_set()
  qapi: Drop redundant, unclean error_is_set()
  hmp: Guard against misuse of hmp_handle_error()
  qga: Use return values instead of error_is_set(errp)
  error: Consistently name Error ** objects errp, and not err
  qmp: Consistently name Error ** objects errp, and not err
  qga: Consistently name Error ** objects errp, and not err
  qmp hmp: Consistently name Error * objects err, and not errp
  pci-assign: assigned_initfn(): set monitor error in common error handler
  pci-assign: propagate errors from assign_intx()
  pci-assign: propagate errors from assign_device()
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 40 insertions, 5 deletions

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index d78921f875..26312d84e8 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -40,6 +40,17 @@ enumeration types and union types.
 Generally speaking, types definitions should always use CamelCase for the type
 names. Command names should be all lower case with words separated by a hyphen.
 
+
+=== Includes ===
+
+The QAPI schema definitions can be modularized using the 'include' directive:
+
+ { 'include': 'path/to/file.json'}
+
+The directive is evaluated recursively, and include paths are relative to the
+file using the directive.
+
+
 === Complex types ===
 
 A complex type is a dictionary containing a single key whose value is a
@@ -49,10 +60,34 @@ example of a complex type is:
  { 'type': 'MyType',
    'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }
 
-The use of '*' as a prefix to the name means the member is optional.  Optional
-members should always be added to the end of the dictionary to preserve
-backwards compatibility.
+The use of '*' as a prefix to the name means the member is optional.
+
+The default initialization value of an optional argument should not be changed
+between versions of QEMU unless the new default maintains backward
+compatibility to the user-visible behavior of the old default.
+
+With proper documentation, this policy still allows some flexibility; for
+example, documenting that a default of 0 picks an optimal buffer size allows
+one release to declare the optimal size at 512 while another release declares
+the optimal size at 4096 - the user-visible behavior is not the bytes used by
+the buffer, but the fact that the buffer was optimal size.
+
+On input structures (only mentioned in the 'data' side of a command), changing
+from mandatory to optional is safe (older clients will supply the option, and
+newer clients can benefit from the default); changing from optional to
+mandatory is backwards incompatible (older clients may be omitting the option,
+and must continue to work).
+
+On output structures (only mentioned in the 'returns' side of a command),
+changing from mandatory to optional is in general unsafe (older clients may be
+expecting the field, and could crash if it is missing), although it can be done
+if the only way that the optional argument will be omitted is when it is
+triggered by the presence of a new input flag to the command that older clients
+don't know to send.  Changing from optional to mandatory is safe.
 
+A structure that is used in both input and output of various commands
+must consider the backwards compatibility constraints of both directions
+of use.
 
 A complex type definition can specify another complex type as its base.
 In this case, the fields of the base type are included as top-level fields
@@ -221,7 +256,7 @@ created code.
 Example:
 
     mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-types.py \
-      --output-dir="qapi-generated" --prefix="example-" < example-schema.json
+      --output-dir="qapi-generated" --prefix="example-" --input-file=example-schema.json
     mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-types.c
     /* AUTOMATICALLY GENERATED, DO NOT MODIFY */
 
@@ -291,7 +326,7 @@ $(prefix)qapi-visit.h: declarations for previously mentioned visitor
 Example:
 
     mdroth@illuin:~/w/qemu2.git$ python scripts/qapi-visit.py \
-        --output-dir="qapi-generated" --prefix="example-" < example-schema.json
+        --output-dir="qapi-generated" --prefix="example-" --input-file=example-schema.json
     mdroth@illuin:~/w/qemu2.git$ cat qapi-generated/example-qapi-visit.c
     /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */