Automatically set PRAGMAs using connection query params (#85)

* Automatically set PRAGMAs using connection query params

Introduce the flexibility to adjust certain PRAGMAs of the SQLite3
connection without having to hardcode those in your codebase (and wait
for compilation).

This allows applications to use `DATABASE_URL` to dynamically fine tune
their SQLite3 configuration.

The change complements `#setup_connection` that offers, via code, the
option to perform queries on setup of each connection.

Only a few PRAGMAs necessary to allow more performant concurrent reads
and reduce write locking.

These pragmas are detected and combined in a single SQL string to reduce
to 1 the number of calls to `sqlite3_exec` C function.

There is no validation of supplied values as SQLite3 automatically
ignores incorrect values for these pragmas.

Closes #84

References:
- https://www.sqlite.org/pragma.html

* Simplify PRAGMA mapping and detection

No longer prefix PRAGMAS with `_`, so the mapping between the real
SQLite3 pragmas and the usage in the URI is more direct.

Use macros instead of case to detect pragmas from URI params and return
those as NamedTuple.

* Update README.md

---------

Co-authored-by: Brian J. Cardiff <bcardiff@gmail.com>

README.md

@@ -48,3 +48,39 @@ end
 
 * `Time` is implemented as `TEXT` column using `SQLite3::DATE_FORMAT_SUBSECOND` format (or `SQLite3::DATE_FORMAT_SECOND` if the text does not contain a dot).
 * `Bool` is implemented as `INT` column mapping `0`/`1` values.
+
+### Setting PRAGMAs
+
+You can adjust certain [SQLite3 PRAGMAs](https://www.sqlite.org/pragma.html)
+automatically when the connection is created by using the query parameters:
+
+```crystal
+require "sqlite3"
+
+DB.open "sqlite3://./data.db?journal_mode=wal&synchronous=normal" do |db|
+  # this database now uses WAL journal and normal synchronous mode
+  # (defaults were `delete` and `full`, respectively)
+end
+```
+
+The following is the list of supported options:
+
+| Name                      | Connection key  |
+|---------------------------|-----------------|
+| [Busy Timeout][pragma-to] | `busy_timeout` |
+| [Cache Size][pragma-cs] | `cache_size` |
+| [Foreign Keys][pragma-fk] | `foreign_keys` |
+| [Journal Mode][pragma-jm] | `journal_mode` |
+| [Synchronous][pragma-sync] | `synchronous` |
+| [WAL autocheckoint][pragma-walck] | `wal_autocheckpoint` |
+
+Please note there values passed using these connection keys are passed
+directly to SQLite3 without check or evaluation. Using incorrect values result
+in no error by the library.
+
+[pragma-to]: https://www.sqlite.org/pragma.html#pragma_busy_timeout
+[pragma-cs]: https://www.sqlite.org/pragma.html#pragma_cache_size
+[pragma-fk]: https://www.sqlite.org/pragma.html#pragma_foreign_keys
+[pragma-jm]: https://www.sqlite.org/pragma.html#pragma_journal_mode
+[pragma-sync]: https://www.sqlite.org/pragma.html#pragma_synchronous
+[pragma-walck]: https://www.sqlite.org/pragma.html#pragma_wal_autocheckpoint

spec/connection_spec.cr

@@ -10,6 +10,14 @@ private def dump(source, target)
   end
 end
 
+private def it_sets_pragma_on_connection(pragma : String, value : String, expected, file = __FILE__, line = __LINE__)
+  it "sets pragma '#{pragma}' to #{expected}", file, line do
+    with_db("#{DB_FILENAME}?#{pragma}=#{value}") do |db|
+      db.scalar("PRAGMA #{pragma}").should eq(expected)
+    end
+  end
+end
+
 describe Connection do
   it "opens a database and then backs it up to another db" do
     with_db do |db|
@@ -68,4 +76,34 @@ describe Connection do
       cnn.scalar("select count(*) from person").should eq(1)
     end
   end
+
+  # adjust busy_timeout pragma (default is 0)
+  it_sets_pragma_on_connection "busy_timeout", "1000", 1000
+
+  # adjust cache_size pragma (default is -2000, 2MB)
+  it_sets_pragma_on_connection "cache_size", "-4000", -4000
+
+  # enable foreign_keys, no need to test off (is the default)
+  it_sets_pragma_on_connection "foreign_keys", "1", 1
+  it_sets_pragma_on_connection "foreign_keys", "yes", 1
+  it_sets_pragma_on_connection "foreign_keys", "true", 1
+  it_sets_pragma_on_connection "foreign_keys", "on", 1
+
+  # change journal_mode (default is delete)
+  it_sets_pragma_on_connection "journal_mode", "delete", "delete"
+  it_sets_pragma_on_connection "journal_mode", "truncate", "truncate"
+  it_sets_pragma_on_connection "journal_mode", "persist", "persist"
+
+  # change synchronous mode (default is 2, FULL)
+  it_sets_pragma_on_connection "synchronous", "0", 0
+  it_sets_pragma_on_connection "synchronous", "off", 0
+  it_sets_pragma_on_connection "synchronous", "1", 1
+  it_sets_pragma_on_connection "synchronous", "normal", 1
+  it_sets_pragma_on_connection "synchronous", "2", 2
+  it_sets_pragma_on_connection "synchronous", "full", 2
+  it_sets_pragma_on_connection "synchronous", "3", 3
+  it_sets_pragma_on_connection "synchronous", "extra", 3
+
+  # change wal_autocheckpoint (default is 1000)
+  it_sets_pragma_on_connection "wal_autocheckpoint", "0", 0
 end

src/sqlite3/connection.cr

@@ -2,10 +2,11 @@ class SQLite3::Connection < DB::Connection
   def initialize(database)
     super
     filename = self.class.filename(database.uri)
-    # TODO maybe enable Flag::URI to parse query string in the uri as additional flags
     check LibSQLite3.open_v2(filename, out @db, (Flag::READWRITE | Flag::CREATE), nil)
     # 2 means 2 arguments; 1 is the code for UTF-8
     check LibSQLite3.create_function(@db, "regexp", 2, 1, nil, SQLite3::REGEXP_FN, nil, nil)
+
+    process_query_params(database.uri)
   rescue
     raise DB::ConnectionRefused.new
   end
@@ -88,4 +89,44 @@ class SQLite3::Connection < DB::Connection
   private def check(code)
     raise Exception.new(self) unless code == 0
   end
+
+  private def process_query_params(uri : URI)
+    return unless query = uri.query
+
+    detected_pragmas = extract_params(query,
+      busy_timeout: nil,
+      cache_size: nil,
+      foreign_keys: nil,
+      journal_mode: nil,
+      synchronous: nil,
+      wal_autocheckpoint: nil,
+    )
+
+    # concatenate all into a single SQL string
+    sql = String.build do |str|
+      detected_pragmas.each do |key, value|
+        next unless value
+        str << "PRAGMA #{key}=#{value};"
+      end
+    end
+
+    check LibSQLite3.exec(@db, sql, nil, nil, nil)
+  end
+
+  private def extract_params(query : String, **default : **T) forall T
+    res = default
+
+    URI::Params.parse(query) do |key, value|
+      {% begin %}
+        case key
+        {% for key in T %}
+        when {{ key.stringify }}
+          res = res.merge({{key.id}}: value)
+        {% end %}
+        end
+      {% end %}
+    end
+
+    res
+  end
 end

src/sqlite3/lib_sqlite3.cr

@@ -86,6 +86,7 @@ lib LibSQLite3
   fun backup_finish = sqlite3_backup_finish(SQLite3Backup) : Code
 
   fun prepare_v2 = sqlite3_prepare_v2(db : SQLite3, zSql : UInt8*, nByte : Int32, ppStmt : Statement*, pzTail : UInt8**) : Int32
+  fun exec = sqlite3_exec(db : SQLite3, zSql : UInt8*, pCallback : Callback, pCallbackArgs : Void*, pzErrMsg : UInt8**) : Int32
   fun step = sqlite3_step(stmt : Statement) : Int32
   fun column_count = sqlite3_column_count(stmt : Statement) : Int32
   fun column_type = sqlite3_column_type(stmt : Statement, iCol : Int32) : ::SQLite3::Type