Add Accountsdb plugin documentations (#21746) (#21799)

Add the public facing documentation about the plugin framework: explaining the interface, how to load plugin and the example PostgreSQL plugin implementation.
Updated the rust documentation for the plugin interfaces for accounts and slot.
This changes are targeted for v1.8. Information about transactions will be updated later.

accountsdb-plugin-interface/src/accountsdb_plugin_interface.rs

@@ -12,16 +12,38 @@ use {
 impl Eq for ReplicaAccountInfo<'_> {}
 
 #[derive(Clone, PartialEq, Debug)]
+/// Information about an account being updated
 pub struct ReplicaAccountInfo<'a> {
+    /// The Pubkey for the account
     pub pubkey: &'a [u8],
+
+    /// The lamports for the account
     pub lamports: u64,
+
+    /// The Pubkey of the owner program account
     pub owner: &'a [u8],
+
+    /// This account's data contains a loaded program (and is now read-only)
     pub executable: bool,
+
+    /// The epoch at which this account will next owe rent
     pub rent_epoch: u64,
+
+    /// The data held in this account.
     pub data: &'a [u8],
+
+    /// A global monotonically increasing atomic number, which can be used
+    /// to tell the order of the account update. For example, when an
+    /// account is updated in the same slot multiple times, the update
+    /// with higher write_version should supersede the one with lower
+    /// write_version.
     pub write_version: u64,
 }
 
+/// A wrapper to future-proof ReplicaAccountInfo handling.
+/// If there were a change to the structure of ReplicaAccountInfo,
+/// there would be new enum entry for the newer version, forcing
+/// plugin implementations to handle the change.
 pub enum ReplicaAccountInfoVersions<'a> {
     V0_0_1(&'a ReplicaAccountInfo<'a>),
 }
@@ -38,28 +60,44 @@ pub enum ReplicaTransactionInfoVersions<'a> {
     V0_0_1(&'a ReplicaTransactionInfo<'a>),
 }
 
+/// Errors returned by plugin calls
 #[derive(Error, Debug)]
 pub enum AccountsDbPluginError {
+    /// Error opening the configuration file; for example, when the file
+    /// is not found or when the validator process has no permission to read it.
     #[error("Error opening config file. Error detail: ({0}).")]
     ConfigFileOpenError(#[from] io::Error),
 
+    /// Error in reading the content of the config file or the content
+    /// is not in the expected format.
     #[error("Error reading config file. Error message: ({msg})")]
     ConfigFileReadError { msg: String },
 
+    /// Error when updating the account.
     #[error("Error updating account. Error message: ({msg})")]
     AccountsUpdateError { msg: String },
 
+    /// Error when updating the slot status
     #[error("Error updating slot status. Error message: ({msg})")]
     SlotStatusUpdateError { msg: String },
 
+    /// Any custom error defined by the plugin.
     #[error("Plugin-defined custom error. Error message: ({0})")]
     Custom(Box<dyn error::Error + Send + Sync>),
 }
 
+/// The current status of a slot
 #[derive(Debug, Clone)]
 pub enum SlotStatus {
+    /// The highest slot of the heaviest fork processed by the node. Ledger state at this slot is
+    /// not derived from a confirmed or finalized block, but if multiple forks are present, is from
+    /// the fork the validator believes is most likely to finalize.
     Processed,
+
+    /// The highest slot having reached max vote lockout.
     Rooted,
+
+    /// The highest slot that has been voted on by supermajority of the cluster, ie. is confirmed.
     Confirmed,
 }
 
@@ -75,6 +113,9 @@ impl SlotStatus {
 
 pub type Result<T> = std::result::Result<T, AccountsDbPluginError>;
 
+/// Defines an AccountsDb plugin, to stream data from the runtime.
+/// AccountsDb plugins must describe desired behavior for load and unload,
+/// as well as how they will handle streamed data.
 pub trait AccountsDbPlugin: Any + Send + Sync + std::fmt::Debug {
     fn name(&self) -> &'static str;
 
@@ -93,6 +134,9 @@ pub trait AccountsDbPlugin: Any + Send + Sync + std::fmt::Debug {
     fn on_unload(&mut self) {}
 
     /// Called when an account is updated at a slot.
+    /// When `is_startup` is true, it indicates the account is loaded from
+    /// snapshots when the validator starts up. When `is_startup` is false,
+    /// the account is updated during transaction processing.
     #[allow(unused_variables)]
     fn update_account(
         &mut self,