Merge bitcoin/bitcoin#20923: signet miner followups

b3c712cb2871e0034898b37454fb6a4e1fc78f84 contrib/signet/miner: remove debug code (Anthony Towns)
297e35159f99d4caa995de29d419aefa4b87630c bitcoin-util: use AddCommand / GetCommand (Anthony Towns)
b6d493fd4d602a67d16d2566466e18091e272fa0 contrib/signet/README.md: Update miner description (Anthony Towns)
e66543827cd4ae9b76ff4c5c6e414c1942c7d727 contrib/signet/miner: Automatic timestamp for first block (Anthony Towns)
a383ce5b4add6cdf4e9974527b609a1147a0d972 contrib/signet/miner: --grind-cmd is required for calibrate (Anthony Towns)
1a45cd2e51214f81442a5f9bf16612100adaf5b8 contrib/signet: Fix typos (Anthony Towns)

Pull request description:

  Followups from #19937

ACKs for top commit:
  laanwj:
    Code review ACK b3c712cb2871e0034898b37454fb6a4e1fc78f84

Tree-SHA512: a1003f9ee3697438114b60872b50f4300c8b52f0d58551566eb61c421d787525807ae75be205dcab2c24358cd568f53260120880109a9d728773405ff987596f

2 files changed, 26 insertions, 31 deletions

diff --git a/contrib/signet/README.md b/contrib/signet/README.md
index 71dc2f9638..706b296c54 100644
--- a/contrib/signet/README.md
+++ b/contrib/signet/README.md
@@ -21,27 +21,28 @@ accept one claim per day. See `--password` above.
 miner
 =====
 
-To mine the first block in your custom chain, you can run:
+You will first need to pick a difficulty target. Since signet chains are primarily protected by a signature rather than proof of work, there is no need to spend as much energy as possible mining, however you may wish to choose to spend more time than the absolute minimum. The calibrate subcommand can be used to pick a target appropriate for your hardware, eg:
 
     cd src/
-    CLI="./bitcoin-cli -conf=mysignet.conf"
-    MINER="..contrib/signet/miner"
+    MINER="../contrib/signet/miner"
     GRIND="./bitcoin-util grind"
-    ADDR=$($CLI -signet getnewaddress)
-    $MINER --cli="$CLI" generate --grind-cmd="$GRIND" --address="$ADDR" --set-block-time=-1
-
-This will mine a block with the current timestamp. If you want to backdate the chain, you can give a different timestamp to --set-block-time.
-
-You will then need to pick a difficulty target. Since signet chains are primarily protected by a signature rather than proof of work, there is no need to spend as much energy as possible mining, however you may wish to choose to spend more time than the absolute minimum. The calibrate subcommand can be used to pick a target, eg:
-
     $MINER calibrate --grind-cmd="$GRIND"
     nbits=1e00f403 for 25s average mining time
 
 It defaults to estimating an nbits value resulting in 25s average time to find a block, but the --seconds parameter can be used to pick a different target, or the --nbits parameter can be used to estimate how long it will take for a given difficulty.
 
-Using the --ongoing parameter will then cause the signet miner to create blocks indefinitely. It will pick the time between blocks so that difficulty is adjusted to match the provided --nbits value.
+To mine the first block in your custom chain, you can run:
 
-    $MINER --cli="$CLI" generate --grind-cmd="$GRIND" --address="$ADDR" --nbits=1e00f403 --ongoing
+    CLI="./bitcoin-cli -conf=mysignet.conf"
+    ADDR=$($CLI -signet getnewaddress)
+    NBITS=1e00f403
+    $MINER --cli="$CLI" generate --grind-cmd="$GRIND" --address="$ADDR" --nbits=$NBITS
+
+This will mine a single block with a backdated timestamp designed to allow 100 blocks to be mined as quickly as possible, so that it is possible to do transactions.
+
+Adding the --ongoing parameter will then cause the signet miner to create blocks indefinitely. It will pick the time between blocks so that difficulty is adjusted to match the provided --nbits value.
+
+    $MINER --cli="$CLI" generate --grind-cmd="$GRIND" --address="$ADDR" --nbits=$NBITS --ongoing
 
 Other options
 -------------
@@ -50,9 +51,11 @@ The --debug and --quiet options are available to control how noisy the signet mi
 
 Instead of specifying --ongoing, you can specify --max-blocks=N to mine N blocks and stop.
 
-Instead of using a single address, a ranged descriptor may be provided instead (via the --descriptor parameter), with the reward for the block at height H being sent to the H'th address generated from the descriptor.
+The --set-block-time option is available to manually move timestamps forward or backward (subject to the rules that blocktime must be greater than mediantime, and dates can't be more than two hours in the future). It can only be used when mining a single block (ie, not when using --ongoing or --max-blocks greater than 1).
+
+Instead of using a single address, a ranged descriptor may be provided via the --descriptor parameter, with the reward for the block at height H being sent to the H'th address generated from the descriptor.
 
-Instead of calculating a specific nbits value, --min-nbits can be specified instead, in which case the mininmum signet difficulty will be targeted.
+Instead of calculating a specific nbits value, --min-nbits can be specified instead, in which case the minimum signet difficulty will be targeted. Signet's minimum difficulty corresponds to --nbits=1e0377ae.
 
 By default, the signet miner mines blocks at fixed intervals with minimal variation. If you want blocks to appear more randomly, as they do in mainnet, specify the --poisson option.
 
@@ -76,5 +79,5 @@ These steps can instead be done explicitly:
       $MINER --cli="$CLI" solvepsbt --grind-cmd="$GRIND" |
       $CLI -signet -stdin submitblock
 
-This is intended to allow you to replace part of the pipeline for further experimentation, if desired.
+This is intended to allow you to replace part of the pipeline for further experimentation (eg, to sign the block with a hardware wallet).
 
diff --git a/contrib/signet/miner b/contrib/signet/miner
index a3fba49d0e..f1360818ae 100755
--- a/contrib/signet/miner
+++ b/contrib/signet/miner
@@ -428,10 +428,13 @@ def do_generate(args):
             action_time = now
             is_mine = True
         elif bestheader["height"] == 0:
-            logging.error("When mining first block in a new signet, must specify --set-block-time")
-            return 1
+            time_delta = next_block_delta(int(bestheader["bits"], 16), bci["bestblockhash"], ultimate_target, args.poisson)
+            time_delta *= 100 # 100 blocks
+            logging.info("Backdating time for first block to %d minutes ago" % (time_delta/60))
+            mine_time = now - time_delta
+            action_time = now
+            is_mine = True
         else:
-
             time_delta = next_block_delta(int(bestheader["bits"], 16), bci["bestblockhash"], ultimate_target, args.poisson)
             mine_time = bestheader["time"] + time_delta
 
@@ -520,12 +523,11 @@ def do_calibrate(args):
         sys.stderr.write("Can only specify one of --nbits or --seconds\n")
         return 1
     if args.nbits is not None and len(args.nbits) != 8:
-        sys.stderr.write("Must specify 8 hex digits for --nbits")
+        sys.stderr.write("Must specify 8 hex digits for --nbits\n")
         return 1
 
     TRIALS = 600 # gets variance down pretty low
     TRIAL_BITS = 0x1e3ea75f # takes about 5m to do 600 trials
-    #TRIAL_BITS = 0x1e7ea75f # XXX
 
     header = CBlockHeader()
     header.nBits = TRIAL_BITS
@@ -533,23 +535,14 @@ def do_calibrate(args):
 
     start = time.time()
     count = 0
-    #CHECKS=[]
     for i in range(TRIALS):
         header.nTime = i
         header.nNonce = 0
         headhex = header.serialize().hex()
         cmd = args.grind_cmd.split(" ") + [headhex]
         newheadhex = subprocess.run(cmd, stdout=subprocess.PIPE, input=b"", check=True).stdout.strip()
-        #newhead = FromHex(CBlockHeader(), newheadhex.decode('utf8'))
-        #count += newhead.nNonce
-        #if (i+1) % 100 == 0:
-        #   CHECKS.append((i+1, count, time.time()-start))
-
-    #print("checks =", [c*1.0 / (b*targ*2**-256) for _,b,c in CHECKS])
 
     avg = (time.time() - start) * 1.0 / TRIALS
-    #exp_count = 2**256 / targ * TRIALS
-    #print("avg =", avg, "count =", count, "exp_count =", exp_count)
 
     if args.nbits is not None:
         want_targ = nbits_to_target(int(args.nbits,16))
@@ -590,7 +583,6 @@ def main():
     generate.add_argument("--nbits", default=None, type=str, help="Target nBits (specify difficulty)")
     generate.add_argument("--min-nbits", action="store_true", help="Target minimum nBits (use min difficulty)")
     generate.add_argument("--poisson", action="store_true", help="Simulate randomised block times")
-    #generate.add_argument("--signcmd", default=None, type=str, help="Alternative signing command")
     generate.add_argument("--multiminer", default=None, type=str, help="Specify which set of blocks to mine (eg: 1-40/100 for the first 40%%, 2/3 for the second 3rd)")
     generate.add_argument("--backup-delay", default=300, type=int, help="Seconds to delay before mining blocks reserved for other miners (default=300)")
     generate.add_argument("--standby-delay", default=0, type=int, help="Seconds to delay before mining blocks (default=0)")
@@ -605,7 +597,7 @@ def main():
         sp.add_argument("--descriptor", default=None, type=str, help="Descriptor for block reward payment")
 
     for sp in [solvepsbt, generate, calibrate]:
-        sp.add_argument("--grind-cmd", default=None, type=str, help="Command to grind a block header for proof-of-work")
+        sp.add_argument("--grind-cmd", default=None, type=str, required=(sp==calibrate), help="Command to grind a block header for proof-of-work")
 
     args = parser.parse_args(sys.argv[1:])