56373: hlc: introduce synthetic flag on timestamps r=nvanbenschoten a=nvanbenschoten

Informs #52745.
Informs #36431.

This commit introduces an 8-bit `flags` field on the hlc timestamp struct. The flags are used to provide details about the timestamp and its meaning. They do not affect the sort order of Timestamps.

The commit then introduces the first flag: SYNTHETIC. As discussed in #52745, a synthetic timestamp is defined as a timestamp that makes no claim about the value of clocks in the system. While standard timestamps are pulled from HLC clocks and indicate that some node in the system has a clock with a reading equal to or above its value, a synthetic timestamp makes no such indication. By avoiding a connection to "real time", synthetic timestamps can be used to write values at a future time and to indicate that observed timestamps do not apply to such writes for the purposes of tracking causality between the write and its observers. Observed timestamps will be a critical part of implementing non-blocking transactions (#52745) and fixing the interaction between observed timestamps and transaction refreshing (#36431).

The original plan was to reserve the high-order bit in the logical portion of a timestamp as a "synthetic bit". This is how I began implementing things, but was turned off for a few reasons. First, it was fairly subtle and seemed too easy to get wrong. Using a separate field is more explicit and avoids a class of bugs. Second, I began to have serious concerns about how the synthetic bit would impact timestamp ordering. Every timestamp comparison would need to mask out the bit or risk being incorrect. This was even true of the LSM custom comparator. This seemed difficult to get right and seemed particularly concerning since we're planning on marking only some of a transaction's committed values as synthetic to fix #36431, so if we weren't careful, we could get atomicity violations. There were also minor backwards compatibility concerns.

But a separate field is more expensive in theory, so we need to be careful. However, it turns out that a separate field is mostly free in each case that we care about. In memory, the separate field is effectively free because the Timestamp struct was previously 12 bytes but was always padded out to 16 bytes when included as a field in any other struct. This means that the flags field is replacing existing padding. Over the wire, the field will not be included when zero and will use a varint encoding when not zero, so again, it is mostly free. In the engine key encoding, the field is also not included when zero, and takes up only 1 byte when non-zero, so it is mostly free.

----

First three commits from #56477.

@sumeerbhola I'm hoping you can take a look at the engine-level changes in the `introduce synthetic flag on timestamps` commit (4th commit as of the time of writing). I think the key encoding added here makes sense, but want to make sure you're on board. One possible concern is that we introduce a new 13-byte suffix, which means that combined with a 4-byte sequence number (see #41720 (comment)), we'd collide with the 17 byte `engineKeyVersionLockTableLen`.

@tbg do you mind being the primary reviewer here? I think you know the most about the motivations for this change and will have a good sense of whether this is the best way to introduce additional state on timestamps.

56437: cli, ui: dismiss release notes signup banner per environment variable r=knz,dhartunian a=nkodali

Previously, the signup banner could only be dismissed manually.
For internal testing purposes, this banner is unnecessary. This
change provides a way to dismiss the signup banner upon start of
a cluster via the cli by setting the environment variable
COCKROACH_UI_RELEASE_NOTES_SIGNUP_DISMISSED=true.

Resolves #46998

Release note: none

56627: sql: rework SHOW REGIONS to SHOW REGIONS FROM CLUSTER r=ajstorm a=otan

Resolves #56331 

Release note (sql change): SHOW REGIONS functionality is now deferred to
SHOW REGIONS FROM CLUSTER.

Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
Co-authored-by: Namrata Kodali <namrata@cockroachlabs.com>
Co-authored-by: Oliver Tan <otan@cockroachlabs.com>

56373: hlc: introduce synthetic flag on timestamps r=nvanbenschoten a=nvanbenschoten

Informs #52745.
Informs #36431.

This commit introduces an 8-bit `flags` field on the hlc timestamp struct. The flags are used to provide details about the timestamp and its meaning. They do not affect the sort order of Timestamps.

The commit then introduces the first flag: SYNTHETIC. As discussed in #52745, a synthetic timestamp is defined as a timestamp that makes no claim about the value of clocks in the system. While standard timestamps are pulled from HLC clocks and indicate that some node in the system has a clock with a reading equal to or above its value, a synthetic timestamp makes no such indication. By avoiding a connection to "real time", synthetic timestamps can be used to write values at a future time and to indicate that observed timestamps do not apply to such writes for the purposes of tracking causality between the write and its observers. Observed timestamps will be a critical part of implementing non-blocking transactions (#52745) and fixing the interaction between observed timestamps and transaction refreshing (#36431).

The original plan was to reserve the high-order bit in the logical portion of a timestamp as a "synthetic bit". This is how I began implementing things, but was turned off for a few reasons. First, it was fairly subtle and seemed too easy to get wrong. Using a separate field is more explicit and avoids a class of bugs. Second, I began to have serious concerns about how the synthetic bit would impact timestamp ordering. Every timestamp comparison would need to mask out the bit or risk being incorrect. This was even true of the LSM custom comparator. This seemed difficult to get right and seemed particularly concerning since we're planning on marking only some of a transaction's committed values as synthetic to fix #36431, so if we weren't careful, we could get atomicity violations. There were also minor backwards compatibility concerns.

But a separate field is more expensive in theory, so we need to be careful. However, it turns out that a separate field is mostly free in each case that we care about. In memory, the separate field is effectively free because the Timestamp struct was previously 12 bytes but was always padded out to 16 bytes when included as a field in any other struct. This means that the flags field is replacing existing padding. Over the wire, the field will not be included when zero and will use a varint encoding when not zero, so again, it is mostly free. In the engine key encoding, the field is also not included when zero, and takes up only 1 byte when non-zero, so it is mostly free.

----

First three commits from #56477.

@sumeerbhola I'm hoping you can take a look at the engine-level changes in the `introduce synthetic flag on timestamps` commit (4th commit as of the time of writing). I think the key encoding added here makes sense, but want to make sure you're on board. One possible concern is that we introduce a new 13-byte suffix, which means that combined with a 4-byte sequence number (see #41720 (comment)), we'd collide with the 17 byte `engineKeyVersionLockTableLen`.

@tbg do you mind being the primary reviewer here? I think you know the most about the motivations for this change and will have a good sense of whether this is the best way to introduce additional state on timestamps.

Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>