db: add `sqlite3-available?'

Loading `db/sqlite3' no longer raises an exception if
the SQLite library isn't found. Instead, `sqlite3-connect'
raises an exception, while `sqlite3-available?' reports
whether it will work.

The dynamic test allows the documentation-help system
to continue to work if SQLite3 is not available. Currently,
though, `raco setup' still insists on using SQLite3 to
build the database of documented tags.

collects/db/main.rkt

@@ -14,7 +14,8 @@
    mysql-guess-socket-path
    mysql-password-hash)]
  ["private/sqlite3/main.rkt"
-  (sqlite3-connect)]
+  (sqlite3-connect
+   sqlite3-available?)]
  ["private/odbc/main.rkt"
   (odbc-connect
    odbc-driver-connect
@@ -71,6 +72,8 @@
         #:use-place boolean?
         #:debug? any/c)
        connection?)]
+ [sqlite3-available?
+  (-> boolean?)]
 
  ;; Duplicates contracts at odbc.rkt
  [odbc-connect

collects/db/private/sqlite3/ffi.rkt

@@ -5,10 +5,14 @@
 (provide (all-from-out "ffi-constants.rkt")
          (protect-out (all-defined-out)))
 
-(define-ffi-definer define-sqlite
+(define sqlite-lib
   (case (system-type)
-    ((windows) (ffi-lib "sqlite3.dll"))
-    (else (ffi-lib "libsqlite3" '("0" #f)))))
+    [(windows) (ffi-lib "sqlite3.dll" #:fail (lambda () #f))]
+    [else (ffi-lib "libsqlite3" '("0" #f) #:fail (lambda () #f))]))
+
+(define-ffi-definer define-sqlite
+  sqlite-lib
+  #:default-make-fail make-not-available)
 
 ; Types
 (define-cpointer-type _sqlite3_database)

collects/db/private/sqlite3/main.rkt

@@ -5,7 +5,8 @@
          "connection.rkt"
          "dbsystem.rkt"
          "ffi.rkt")
-(provide sqlite3-connect)
+(provide sqlite3-connect
+         sqlite3-available?)
 
 (define (sqlite3-connect #:database path
                          #:mode [mode 'read/write]
@@ -53,3 +54,6 @@
   (class place-proxy-connection%
     (super-new)
     (define/override (get-dbsystem) dbsystem)))
+
+(define (sqlite3-available?)
+  (and sqlite-lib #t))

collects/db/scribblings/connect.scrbl

@@ -247,6 +247,13 @@ Base connections are made using the following functions.
      (new connection%)]]
 }
 
+
+@defproc[(sqlite3-available?) boolean?]{
+
+Reports whether the SQLite native library is found, in which case
+@racket[sqlite3-connect] works, otherwise it raises an exception.}
+
+
 @defproc[(odbc-connect [#:dsn dsn string?]
                        [#:user user (or/c string? #f) #f]
                        [#:password password (or/c string? #f) #f]
@@ -742,10 +749,9 @@ Provides only @racket[mysql-connect] and
 
 @defmodule*/no-declare[(db/sqlite3)]
 
-Provides only @racket[sqlite3-connect]. In contrast to
-@racketmodname[db], this module immediately attempts to
-load the SQLite native library when required, and it raises an
-exception if it cannot be found.
+Provides @racket[sqlite3-connect] plus @racket[sqlite3-available?]. When
+the SQLite native library cannot be found, @racket[sqlite3-connect]
+raises an exception.
 
 @defmodule*/no-declare[(db/odbc)]
 

collects/db/sqlite3.rkt

@@ -12,4 +12,6 @@
         #:busy-retry-delay (and/c rational? (not/c negative?))
         #:use-place any/c
         #:debug? any/c)
-       connection?)])
+       connection?)]
+ [sqlite3-available?
+  (-> boolean?)])

collects/setup/doc-db.rkt

@@ -8,7 +8,8 @@
          doc-db-key->path
          doc-db-file->connection)
 
-(define (doc-db-available?) #t)
+(define (doc-db-available?)
+  (sqlite3-available?))
 
 (define (doc-db-file->connection db-file)
   (sqlite3-connect #:database db-file))