Fixes for SQLite concurrency issues

In some busy workloads various SQLite operations would fail due to
"locked tables"; most of this is related to the session code:

- On every pageview a new session is either read (OK) or written (LOCK!)
- At the same time, the cron may be writing data to the db (LOCK!)

On smaller instances this isn't much of an issue since everything is
fast enough to not lock for too long, but on longer instances this can
be a problem.

Setting SetMaxOpenConns(1) solves this by limiting the connections
writing to the database.

Also set the default journal mode to WAL, which should give better
performance. Both of this is done in the zgo.at/zdb update.

Add "goatcounter help db" to document database usage a bit better.

cmd/goatcounter/create.go

@@ -26,25 +26,25 @@ Create a new site and user.
 
 Required flags:
 
-  -domain         Domain to host e.g. "stats.example.com". The site will be
-                  available on this domain only, so "stats.example.com" won't be
-                  available on "localhost".
+  -domain      Domain to host e.g. "stats.example.com". The site will be
+               available on this domain only, so "stats.example.com" won't be
+               available on "localhost".
 
-  -email          Your email address. Will be required to login.
+  -email       Your email address. Will be required to login.
 
 Other flags:
 
-  -password      Password to log in; will be asked interactively if omitted.
+  -password    Password to log in; will be asked interactively if omitted.
 
-  -parent        Parent site; either as ID or domain.
+  -parent      Parent site; either as ID or domain.
 
-  -db            Database connection string. Use "sqlite://<dbfile>" for SQLite,
-                 or "postgres://<connect string>" for PostgreSQL
-                 Default: sqlite://db/goatcounter.sqlite3
+  -db          Database connection: "sqlite://<file>" or "postgres://<connect>"
+               See "goatcounter help db" for detailed documentation. Default:
+               sqlite://db/goatcounter.sqlite3?_busy_timeout=200&_journal_mode=wal&cache=shared
 
-  -createdb      Create the database if it doesn't exist yet; only for SQLite.
+  -createdb    Create the database if it doesn't exist yet; only for SQLite.
 
-  -debug         Modules to debug, comma-separated or 'all' for all modules.
+  -debug       Modules to debug, comma-separated or 'all' for all modules.
 `
 
 func create() (int, error) {

cmd/goatcounter/help.go

@@ -16,6 +16,66 @@ Show help; use "help commands" to dispay detailed help for a command, or "help
 all" to display everything.
 `
 
+const helpDatabase = `
+GoatCounter can use SQLite and PostgreSQL. All commands accept the -db flag to
+customize the database connection string.
+
+You can select a database engine by using "sqlite://[..]" for SQLite, or
+"postgresql://[..]" (or "postgres://[..]") for PostgreSQL.
+
+There are no plans to support other database engines such as MySQL/MariaDB.
+
+SQLite
+
+    This is the default database engine as it has no dependencies, and for most
+    small to medium usage it should be more than fast enough.
+
+    The SQLite connection string is usually just a filename, optionally prefixed
+    with "file:". Parameters can be added as a URL query string after a ?:
+
+        -db 'sqlite://mydb.sqlite?param=value&other=value'
+
+    See the go-sqlite3 documentation for a list of supported parameters:
+    https://github.com/mattn/go-sqlite3/#connection-string
+
+    _journal_mode=wal is always added unless explicitly overridden. Generally
+    speaking using a Write-Ahead-Log is more suitable for GoatCounter than the
+    default DELETE journaling.
+
+    The database is automatically created for the "serve" command, but you need
+    to add -createdb to any other commands to create the database. This is to
+    prevent accidentally operating on the wrong (new) database.
+
+PostgreSQL
+
+    PostgreSQL provides better performance for large instances. If you have
+    millions of pageviews then PostgreSQL is probably a better choice.
+
+    The PostgreSQL connection string can either be as "key=value" or as an URL;
+    the following are identical:
+
+        -db 'postgresql://user=pqgotest dbname=pqgotest sslmode=verify-full'
+        -db 'postgresql://pqgotest:password@localhost/pqgotest?sslmode=verify-full'
+
+    See the pq documentation for a list of supported parameters:
+    https://pkg.go.dev/github.com/lib/pq?tab=doc#hdr-Connection_String_Parameters
+
+    You may want to consider lowering the "seq_page_cost" parameter; the query
+    planner tends to prefer seq scans instead of index scans for some operations
+    with the default of 4, which is much slower.
+
+    I found that 0.5 is a fairly good setting, you can set it in your
+    postgresql.conf file, or just for one database with:
+
+        alter database goatcounter set seq_page_cost=.5
+
+    The database isn't automatically created for PostgreSQL, you'll have to
+    manually create it first:
+
+        createdb goatcounter
+        psql goatcounter < ./db/schema.pgsql
+`
+
 func help() (int, error) {
 	if len(os.Args) == 2 {
 		fmt.Fprint(stdout, usage[""])
@@ -24,7 +84,12 @@ func help() (int, error) {
 
 	if os.Args[2] == "all" {
 		fmt.Fprint(stdout, usage[""], "\n")
-		for _, h := range []string{"help", "version", "migrate", "serve", "create", "reindex", "monitor"} {
+		for _, h := range []string{
+			"help", "version",
+			"migrate", "create", "serve",
+			"reindex", "monitor",
+			"database",
+		} {
 			head := fmt.Sprintf("─── Help for %q ", h)
 			fmt.Fprintf(stdout, "%s%s\n\n", head, strings.Repeat("─", 80-utf8.RuneCountInString(head)))
 			fmt.Fprint(stdout, usage[h], "\n")

cmd/goatcounter/main.go

@@ -35,14 +35,16 @@ var (
 )
 
 var usage = map[string]string{
-	"":        usageTop,
-	"help":    usageHelp,
-	"serve":   usageServe,
-	"create":  usageCreate,
-	"migrate": usageMigrate,
-	"saas":    usageSaas,
-	"reindex": usageReindex,
-	"monitor": usageMonitor,
+	"":         usageTop,
+	"help":     usageHelp,
+	"serve":    usageServe,
+	"create":   usageCreate,
+	"migrate":  usageMigrate,
+	"saas":     usageSaas,
+	"reindex":  usageReindex,
+	"monitor":  usageMonitor,
+	"database": helpDatabase,
+	"db":       helpDatabase,
 
 	"version": `
 Show version and build information. This is printed as key=value, separated by
@@ -62,17 +64,20 @@ Usage: goatcounter [command] [flags]
 
 Commands:
 
-  help        Show help; use "help <topic>" or "help all" for more details.
-  version     Show version and build information and exit.
-  migrate     Run database migrations.
-  create      Create a new site and user.
-  serve       Start HTTP server.
+  help         Show help; use "help <topic>" or "help all" for more details.
+  version      Show version and build information and exit.
+  migrate      Run database migrations.
+  create       Create a new site and user.
+  serve        Start HTTP server.
 
 Advanced commands:
 
-  reindex     Re-create the cached statistics (*_stats tables) from the hits.
-              This is generally rarely needed and mostly a development tool.
-  monitor     Monitor for pageviews.
+  reindex      Recreate the index tables (*_stats, *_count) from the hits.
+  monitor      Monitor for pageviews.
+
+Extra help topics:
+
+  db           Documentation on the -db flag.
 
 See "help <topic>" for more details for the command.
 `

cmd/goatcounter/migrate.go

@@ -22,13 +22,13 @@ Run database migrations and exit.
 
 Flags:
 
-  -db            Database connection string. Use "sqlite://<dbfile>" for SQLite,
-                 or "postgres://<connect string>" for PostgreSQL
-                 Default: sqlite://db/goatcounter.sqlite3
+  -db          Database connection: "sqlite://<file>" or "postgres://<connect>"
+               See "goatcounter help db" for detailed documentation. Default:
+               sqlite://db/goatcounter.sqlite3?_busy_timeout=200&_journal_mode=wal&cache=shared
 
-  -createdb      Create the database if it doesn't exist yet; only for SQLite.
+  -createdb    Create the database if it doesn't exist yet; only for SQLite.
 
-  -debug         Modules to debug, comma-separated or 'all' for all modules.
+  -debug       Modules to debug, comma-separated or 'all' for all modules.
 
 Positional arguments are names of database migrations, either as just the name
 ("2020-01-05-2-foo") or as the file path ("./db/migrate/sqlite/2020-01-05-2-foo.sql").

cmd/goatcounter/monitor.go

@@ -18,15 +18,15 @@ log if it's 0.
 
 Flags:
 
-  -db            Database connection string. Use "sqlite://<dbfile>" for SQLite,
-                 or "postgres://<connect string>" for PostgreSQL
-                 Default: sqlite://db/goatcounter.sqlite3
+  -db          Database connection: "sqlite://<file>" or "postgres://<connect>"
+               See "goatcounter help db" for detailed documentation. Default:
+               sqlite://db/goatcounter.sqlite3?_busy_timeout=200&_journal_mode=wal&cache=shared
 
-  -debug         Modules to debug, comma-separated or 'all' for all modules.
+  -debug       Modules to debug, comma-separated or 'all' for all modules.
 
-  -period        Check every n seconds. Default: 120.
+  -period      Check every n seconds. Default: 120.
 
-  -site          Limit the check to just one site; makes the query faster.
+  -site        Limit the check to just one site; makes the query faster.
 `
 
 func monitor() (int, error) {

cmd/goatcounter/reindex.go

@@ -43,28 +43,28 @@ Avoiding race conditions
 
 Flags:
 
-  -db            Database connection string. Use "sqlite://<dbfile>" for SQLite,
-                 or "postgres://<connect string>" for PostgreSQL
-                 Default: sqlite://db/goatcounter.sqlite3
+  -db          Database connection: "sqlite://<file>" or "postgres://<connect>"
+               See "goatcounter help db" for detailed documentation. Default:
+               sqlite://db/goatcounter.sqlite3?_busy_timeout=200&_journal_mode=wal&cache=shared
 
-  -debug         Modules to debug, comma-separated or 'all' for all modules.
+  -debug       Modules to debug, comma-separated or 'all' for all modules.
 
-  -pause         Number of seconds to pause after each day, to give the server
-                 some breathing room on large sites. Default: 0.
+  -pause       Number of seconds to pause after each day, to give the server
+               some breathing room on large sites. Default: 0.
 
-  -since         Reindex only statistics since this date instead of all of them;
-                 as year-month-day in UTC.
+  -since       Reindex only statistics since this date instead of all of them;
+               as year-month-day in UTC.
 
-  -to            Reindex only statistics up to and including this day; as
-                 year-month-day in UTC. The default is yesterday.
+  -to          Reindex only statistics up to and including this day; as
+               year-month-day in UTC. The default is yesterday.
 
-  -table         Which tables to reindex: hit_stats, hit_counts, browser_stats,
-                 system_stats, location_stats, ref_counts, size_stats, or all
-                 (default).
+  -table       Which tables to reindex: hit_stats, hit_counts, browser_stats,
+               system_stats, location_stats, ref_counts, size_stats, or all
+               (default).
 
-  -site          Only reindex this site ID. Default is to reindex all.
+  -site        Only reindex this site ID. Default is to reindex all.
 
-  -quiet         Don't print progress.
+  -quiet       Don't print progress.
 `
 
 func reindex() (int, error) {

cmd/goatcounter/serve.go

@@ -33,85 +33,83 @@ with -dev.
 
 Flags:
 
-  -static        Serve static files from a diffent domain, such as a CDN or
-                 cookieless domain. Default: not set.
+  -static      Serve static files from a diffent domain, such as a CDN or
+               cookieless domain. Default: not set.
 
-  -port          Port your site is publicly accessible on. Only needed if it's
-                 not 80 or 443.
+  -port        Port your site is publicly accessible on. Only needed if it's
+               not 80 or 443.
 ` + serveAndSaasFlags
 
 const serveAndSaasFlags = `
-  -db            Database connection string. Use "sqlite://<dbfile>" for SQLite,
-                 or "postgres://<connect string>" for PostgreSQL
-                 Default: sqlite://db/goatcounter.sqlite3
+  -db          Database connection: "sqlite://<file>" or "postgres://<connect>"
+               See "goatcounter help db" for detailed documentation. Default:
+               sqlite://db/goatcounter.sqlite3?_busy_timeout=200&_journal_mode=wal&cache=shared
 
-  -listen        Address to listen on. Default: localhost:8081
+  -listen      Address to listen on. Default: localhost:8081
 
-  -dev           Start in "dev mode".
+  -dev         Start in "dev mode".
 
-  -tls           Serve over tls. This is a comma-separated list with any of:
+  -tls         Serve over tls. This is a comma-separated list with any of:
 
-                   none              Don't serve any TLS.
+                 none              Don't serve any TLS.
 
-                   path/to/file.pem  TLS certificate and keyfile, in one file.
+                 path/to/file.pem  TLS certificate and keyfile, in one file.
 
-                   acme[:cache]      Create TLS certificates with ACME, this can
-                                     optionally followed by a : and a cache
-                                     directory name (default: acme-secrets).
+                 acme[:cache]      Create TLS certificates with ACME, this can
+                                   optionally followed by a : and a cache
+                                   directory name (default: acme-secrets).
 
-                   tls               Accept TLS connections on -listen.
+                 tls               Accept TLS connections on -listen.
 
-                   rdr               Redirect port 80 to the -listen port. ACME
-                                     verification requires the server to be
-                                     available on port 80.
+                 rdr               Redirect port 80 to the -listen port. ACME
+                                   verification requires the server to be
+                                   available on port 80.
 
-                 Examples:
+               Examples:
 
-                   acme                       Create ACME certs but serve HTTP,
-                                              useful when serving behind proxy
-                                              which can use the certs.
+                 acme                       Create ACME certs but serve HTTP,
+                                            useful when serving behind proxy
+                                            which can use the certs.
 
-                   acme:/home/gc/.acme        As above, but with custom cache dir.
+                 acme:/home/gc/.acme        As above, but with custom cache dir.
 
-                   ./example.com.pem,tls,rdr  Always use the certificate in the
-                                              file, serve over TLS, and redirect
-                                              port 80.
+                 ./example.com.pem,tls,rdr  Always use the certificate in the
+                                            file, serve over TLS, and redirect
+                                            port 80.
 
-                 Default: "acme,tls,rdr", blank when -dev is given.
+               Default: "acme,tls,rdr", blank when -dev is given.
 
-  -smtp          SMTP server, as URL (e.g. "smtp://user:pass@server").
+  -smtp        SMTP server, as URL (e.g. "smtp://user:pass@server").
 
-                 A special value of "stdout" means no emails will be sent and
-                 emails will be printed to stdout only. This is the default.
+               A special value of "stdout" means no emails will be sent and
+               emails will be printed to stdout only. This is the default.
 
-                 If this is blank emails will be sent without using a relay;
-                 this should work fine, but deliverability will usually be worse
-                 (i.e. it will be more likely to end up in the spam box). This
-                 usually requires rDNS properly set up, and GoatCounter will
-                 *not* retry on errors. Using stdout, a local smtp relay, or a
-                 mailtrap.io box is probably better unless you really know what
-                 you're doing.
+               If this is blank emails will be sent without using a relay; this
+               should work fine, but deliverability will usually be worse (i.e.
+               it will be more likely to end up in the spam box). This usually
+               requires rDNS properly set up, and GoatCounter will *not* retry
+               on errors. Using stdout, a local smtp relay, or a mailtrap.io box
+               is probably better unless you really know what you're doing.
 
-  -email-from    From: address in emails. Default: <user>@<hostname>
+  -email-from  From: address in emails. Default: <user>@<hostname>
 
-  -errors        What to do with errors; they're always printed to stderr.
+  -errors      What to do with errors; they're always printed to stderr.
 
-                   mailto:to_addr[,from_addr]  Email to this address; the
-                                               from_addr is optional and sets
-                                               the From: address. The default is
-                                               to use the same as the to_addr.
+                 mailto:to_addr[,from_addr]  Email to this address; the
+                                             from_addr is optional and sets the
+                                             From: address. The default is to
+                                             use the same as the to_addr.
+               Default: not set.
 
-                 Default: not set.
+  -debug       Modules to debug, comma-separated or 'all' for all modules.
 
-  -debug         Modules to debug, comma-separated or 'all' for all modules.
-
-  -automigrate   Automatically run all pending migrations on startup.
+  -automigrate Automatically run all pending migrations on startup.
 
 Environment:
 
-  TMPDIR         Directory for temporary files; only used to store CSV exports
-                 at the moment. On Windows it will use the first non-empty value
-                 of %TMP%, %TEMP%, and %USERPROFILE%.
+  TMPDIR       Directory for temporary files; only used to store CSV exports at
+               the moment. On Windows it will use the first non-empty value of
+               %TMP%, %TEMP%, and %USERPROFILE%.
 `
 
 func serve() (int, error) {