pg: Add transaction & subtransaction support

Was expecting the implementation of this to get overly complicated and
brittle, but using a counter-based cookie and doing parts of it in Perl
made it pretty easy actually.  Pretty happy with how this turned out so
far.

TODO: documentation -.-

c/libpq.h

@@ -13,6 +13,8 @@ typedef enum {
 } ExecStatusType;
 typedef enum { PQERRORS_TERSE, PQERRORS_DEFAULT, PQERRORS_VERBOSE, PQERRORS_SQLSTATE } PGVerbosity;
 typedef enum { PQSHOW_CONTEXT_NEVER, PQSHOW_CONTEXT_ERRORS, PQSHOW_CONTEXT_ALWAYS } PGContextVisibility;
+typedef enum { CONNECTION_OK, CONNECTION_BAD } ConnStatusType; /* There's more, but they're irrelevant to us */
+typedef enum { PQTRANS_IDLE, PQTRANS_ACTIVE, PQTRANS_INTRANS, PQTRANS_INERROR, PQTRANS_UNKNOWN } PGTransactionStatusType;
 
 #define PG_DIAG_SEVERITY        'S'
 #define PG_DIAG_SEVERITY_NONLOCALIZED 'V'
@@ -62,8 +64,9 @@ typedef enum { PQSHOW_CONTEXT_NEVER, PQSHOW_CONTEXT_ERRORS, PQSHOW_CONTEXT_ALWAY
     X(PQresultStatus, ExecStatusType, const PGresult *) \
     X(PQresultVerboseErrorMessage, char *, const PGresult *, PGVerbosity, PGContextVisibility) \
     X(PQserverVersion, int, const PGconn *) \
-    X(PQstatus, int, const PGconn *) \
+    X(PQstatus, ConnStatusType, const PGconn *) \
     X(PQtrace, void, PGconn *, FILE *) \
+    X(PQtransactionStatus, PGTransactionStatusType, const PGconn *) \
     X(PQuntrace, void, PGconn *)
 
 #define X(n, r, ...) static r (*n)(__VA_ARGS__);

c/pgconn.c

@@ -2,6 +2,7 @@ typedef struct {
     SV *self;
     PGconn *conn;
     UV prep_counter;
+    UV cookie; /* currently active transaction object; 0 = none active */
 } fupg_conn;
 
 
@@ -71,7 +72,7 @@ static void fupg_result_croak(PGresult *r, const char *action, const char *query
 
 static SV *fupg_connect(pTHX_ const char *str) {
     PGconn *conn = PQconnectdb(str);
-    if (PQstatus(conn) != 0) {
+    if (PQstatus(conn) != CONNECTION_OK) {
         SV *sv = fupg_conn_errsv(conn, "connect");
         PQfinish(conn);
         croak_sv(sv);
@@ -82,6 +83,17 @@ static SV *fupg_connect(pTHX_ const char *str) {
     return fupg_selfobj(c, "FU::PG::conn");
 }
 
+static const char *fupg_status(fupg_conn *c) {
+    if (PQstatus(c->conn) == CONNECTION_BAD) return "bad";
+    switch (PQtransactionStatus(c->conn)) {
+        case PQTRANS_IDLE: return c->cookie ? "txn_done" : "idle";
+        case PQTRANS_ACTIVE: return "active"; /* can't happen, we don't do async */
+        case PQTRANS_INTRANS: return "txn_idle";
+        case PQTRANS_INERROR: return "txn_error";
+        default: return "unknown";
+    }
+}
+
 static void fupg_destroy(fupg_conn *c) {
     PQfinish(c->conn);
     safefree(c);
@@ -112,6 +124,26 @@ static SV *fupg_exec(pTHX_ fupg_conn *c, const char *sql) {
     return ret;
 }
 
+/* Validate a FU::PG::txn object and extract the connection */
+static fupg_conn *fupg_get_transaction(pTHX_ SV *sv) {
+    if (!sv_derived_from(sv, "FU::PG::txn")) goto invalid;
+    sv = SvRV(sv);
+    if (SvTYPE(sv) != SVt_PVAV) goto invalid;
+    AV *av = (AV *)sv;
+
+    SV **v = av_fetch(av, 0, 0);
+    if (!v || !*v) goto invalid;
+    fupg_conn *c = (fupg_conn *)SvIVX(SvRV(*v));
+
+    v = av_fetch(av, 1, 0);
+    if (!v || !*v) goto invalid;
+    if (!SvOK(*v)) croak("Invalid attempt to run a query on a transaction that has already finished");
+    if (c->cookie != SvUV(*v)) croak("Invalid cross-transaction operation");
+    return c;
+invalid:
+    croak("invalid transaction object");
+}
+
 /* Read a Perl value from a PGresult.
  * Currently assumes text format and just creates a PV. */
 static SV *fupg_val(pTHX_ const PGresult *r, int row, int col) {
@@ -123,6 +155,7 @@ typedef struct {
     /* Set in $conn->q() */
     SV *self;
     fupg_conn *conn; /* has a refcnt on conn->self */
+    UV cookie;
     char *query;
     SV **bind;
     int bindn;
@@ -139,6 +172,7 @@ typedef struct {
 static SV *fupg_q(pTHX_ fupg_conn *c, const char *query, I32 ax, I32 argc) {
     fupg_st *st = safecalloc(1, sizeof(fupg_st));
     st->conn = c;
+    st->cookie = c->cookie;
     SvREFCNT_inc(c->self);
 
     st->query = savepv(query);
@@ -249,7 +283,6 @@ static void fupg_st_execute(pTHX_ fupg_st *st) {
     PGresult *r = PQexecPrepared(st->conn->conn, st->name, st->paramn, (const char * const*)st->param, NULL, NULL, 0);
     if (!r) fupg_conn_croak(st->conn , "exec");
     switch (PQresultStatus(r)) {
-        case PGRES_EMPTY_QUERY:
         case PGRES_COMMAND_OK:
         case PGRES_TUPLES_OK: break;
         default: fupg_result_croak(r, "exec", st->query);
@@ -334,5 +367,4 @@ static void fupg_st_destroy(fupg_st *st) {
 
 /* TODO: $st->alla, allh, flat, kvv, kva, kvh */
 /* TODO: Prepared statement caching */
-/* TODO: Transactions */
 /* TODO: Binary format fetching & type handling */