Code review comment for ~dirk.zimoch/epics-base:fix_zero_size_arrays

My changes:

commit 297f04bddc6765d7fc2b5941e16054b448fc994e
Author: Andrew Johnson <email address hidden>
Date: Fri Oct 30 13:37:50 2020 -0500

    Make dbgf display something for an empty array

    Also significantly expands on Dirk's Release Notes entries.

diff --git a/documentation/RELEASE_NOTES.md b/documentation/RELEASE_NOTES.md
index 0a54dc67f..f21fbdcec 100644
--- a/documentation/RELEASE_NOTES.md
+++ b/documentation/RELEASE_NOTES.md
@@ -17,42 +17,84 @@ should also be read to understand what has changed since earlier releases.

 <!-- Insert new items immediately below here ... -->

-### Support for empty arrays
+### Support for zero-length arrays

-Several problems with empty arrays have been fixed.
+Several modifications have been made to properly support zero-length
+array values inside the IOC and over Channel Access. Some of these changes
+may affect external code that interfaces with the IOC, either directly or
+over the CA client API so we recommend thorough testing of any external
+code that handles array fields when upgrading to this release.

-#### Changed dbr_size_n(TYPE,COUNT) macro
+Since these changes affect the Channel Access client-side API they will
+require rebuilding any CA Gateways against this version or Base to
+properly handle zero-length arrays. The `caget`, `caput` and `camonitor`
+client programs are known to work with empty arrays as long as they were
+built with this or a later version of EPICS.

-When called with COUNT=0 the macro no longer returns the number of bytes
+#### Change to the db_access.h `dbr_size_n(TYPE, COUNT)` macro
+
+When called with COUNT=0 this macro no longer returns the number of bytes
 required for a scalar (1 element) but for an empty array (0 elements).
-Make sure you don't call it with COUNT=0 when you really mean COUNT=1.
+Make sure code that uses this doesn't call it with COUNT=0 when it really
+means COUNT=1.
+
+Note that the db_access.h header file is included by cadef.h so the change
+can impact Channel Access client programs that use this macro.
+
+#### Channel Access support for zero-length arrays
+
+The `ca_array_put()` and `ca_array_put_callback()` routines now accept an
+element count of zero, and will write a zero-length array to the PV if
+possible. No error will be raised if the target is a scalar field though,
+and the field's value will not be changed.
+
+The `ca_array_get_callback()` and `ca_create_subscription()` routines
+still accept a count of zero to mean fetch as many elements as the PV
+currently holds.
+
+Client programs should be prepared for the `count` fields of any
+`struct event_handler_args` or `struct exception_handler_args` passed to
+their callback routines to be zero.

 #### Array records

-The soft supports of array records aai, waveform, and subArray as well as
-the aSub record type have been fixed to correctly report 0 elements read
-when reading empty arrays from an input link.
+The soft device support for the array records aai, waveform, and subArray
+as well as the aSub record type now correctly report reading 0 elements
+when getting an empty array from an input link.

 #### Array support for dbpf

-The dbpf function now accepts arrays, including empty arrays as a JSON string.
+The dbpf command now accepts array values, including empty arrays, when
+provided as a JSON string. This must be enclosed in quotes so the iocsh
+argument parser sees the JSON as a single argument:

-#### Scalar records reading from empty arrays
+```
+epics> dbpf wf10:i32 '[1, 2, 3, 4, 5]'
+DBF_LONG[5]: 1 = 0x1 2 = 0x2 3 = 0x3 4 = 0x4 5 = 0x5
+```

-Records reading scalar fields from empty arrays are now set to INVALID/LINK
-alarm status.
-Links have to call dbGetLink with pnRequest=NULL to be recognized as requests
-for scalars.
-This changes the semantics of pnRequest=NULL. It is now different from
-requesting up to 1 array element, which may return a valid empty array.
+#### Reading empty arrays as scalar values

-### Writing empty arrays to scalar records
+Record links that get a scalar value from an array that is currently
+empty will cause the record that has the link field to be set to an
+`INVALID/LINK` alarm status.
+The record code must call `dbGetLink()` with `pnRequest=NULL` for it to
+be recognized as a request for a scalar value though.

-Witing an empty array to a scalar field now sets the target record to
-INVALID/LINK alarm but does not modify the value. Before, the value used
-to be set to 0 (without any alarm).
-A target field needs to have the SPC_DBADDR tag to be recognized as an array
-field and the record support must define a put_array_info method.
+This changes the semantics of passing `pnRequest=NULL` to `dbGetLink()`,
+which now behaves differently than passing it a pointer to a long integer
+containing the value 1, which was previously equivalent.
+The latter can successfully fetch a zero-element array without triggering
+a LINK alarm.
+
+#### Writing empty arrays to scalar fields
+
+Record links that put a zero-element array into a scalar field will now set
+the target record to `INVALID/LINK` alarm without changing the field's value.
+Previously the field was set to 0 in this case (with no alarm).
+The target field must be marked as `special(SPC_DBADDR)` to be recognized
+as an array field, and its record support must define a `put_array_info()`
+routine.

 ### Add registerAllRecordDeviceDrivers()

diff --git a/modules/database/src/ioc/db/dbTest.c b/modules/database/src/ioc/db/dbTest.c
index d4ac1d792..6106027a8 100644
--- a/modules/database/src/ioc/db/dbTest.c
+++ b/modules/database/src/ioc/db/dbTest.c
@@ -954,13 +954,13 @@ static void printBuffer(
     }

     /* Now print values */
- if (no_elements == 0)
- return;
-
     if (no_elements == 1)
         sprintf(pmsg, "DBF_%s: ", dbr[dbr_type]);
- else
+ else {
         sprintf(pmsg, "DBF_%s[%ld]: ", dbr[dbr_type], no_elements);
+ if (no_elements == 0)
+ strcat(pmsg, "(empty)");
+ }
     dbpr_msgOut(pMsgBuff, tab_size);

     if (status != 0) {